const cleanups in instructions.c

* libguile/instructions.c (fetch_rtl_instruction_table)
  (fetch_instruction_table): Make the returned table const.  Adapt
  callers.
  (scm_rtl_instruction_list): Fix comment about format of return value.
linker string tables are stateful objects

* module/system/vm/linker.scm (make-string-table): Rework to be a
  stateful object instead of a function object.  Works better in this
  case.  Adapt users.
  (string-table-intern!): Rename from string-table-intern, and just
  return the index of the string.
  (link-string-table!): Rename from link-string-table, and set a flag to
  prevent interning strings after linking, as that's not going to work
  well.

* module/language/objcode/elf.scm (bytecode->elf): Adapt.
Add RTL assembler

* module/Makefile.am:
* module/system/vm/assembler.scm: New module, implementing an assembler
  for RTL.

* test-suite/Makefile.am:
* test-suite/tests/rtl.test: New test suite.

* module/system/vm/elf.scm (make-elf-symbol*): Add constructor; export
  as make-elf-symbol.
  (elf-symbol-len): New export.
  (write-elf32-symbol, write-elf64-symbol): New helpers.
  (write-elf-symbol): New export.
Add runtime support for reading debug information from ELF

* module/Makefile.am:
* module/system/vm/debug.scm: New module.

* module/system/vm/elf.scm (elf-section-by-name): New helper.
  (elf-symbol-table-len): New helper.

* test-suite/tests/rtl.test: Add test for finding debug info.
move procedure-name and procedure-source to procprop.c

* libguile/procprop.h:
* libguile/procprop.c (scm_procedure_name, scm_procedure_source): Move
  these functions here, from debug.[ch].
RTL programs print with their name

* libguile/print.c (iprin1): Use scm_i_program_print for RTL programs
  too.

* libguile/procprop.c (scm_procedure_name): For RTL programs, call
  scm_i_rtl_program_name if there is no override.

* libguile/programs.h:
* libguile/programs.c (scm_i_rtl_program_name): New helper, dispatches
  to (system vm program).
  (scm_i_program_print): For RTL programs, the fallback prints the code
  pointer too.

* module/system/vm/program.scm (rtl-program-name): Use the debug info to
  get an RTL program name.
  (write-program): Work with RTL programs too.

* test-suite/tests/rtl.test ("procedure name"): Add test.
Add RTL disassembler

* module/Makefile.am:
* module/system/vm/disassembler.scm: New module.

* module/system/repl/command.scm (disassemble): Work with RTL programs.
begin-program takes properties alist

* module/system/vm/assembler.scm (assert-match): New helper macro to
  check argument types.
  (<meta>): Add properties field.  Rename name field to "label" to
  indicate that it should be unique.
  (make-meta, meta-name): New helpers.
  (begin-program): Take additional properties argument.
  (emit-init-constants): Adapt to begin-program change.
  (link-symtab): Allow for anonymous procedures.

* test-suite/tests/rtl.test: Adapt tests.
add procedure prelude macro-instructions

* module/system/vm/assembler.scm (pack-flags): New helper.
  (standard-prelude, opt-prelude, kw-prelude): New macro-instructions.

* test-suite/tests/rtl.test: Update tests to use standard-prelude.
Beginnings of tracking of procedure arities in assembler

* module/system/vm/assembler.scm (<meta>, <arity>): Assembler now tracks
  arities of a function.
  (begin-standard-arity, begin-opt-arity, begin-kw-arity, end-arity):
  New macro-assemblers.

* test-suite/tests/rtl.test: Adapt all tests to use begin-standard-arity
  and end-arity.
RTL assembler writes arities information into separate section.

* module/system/vm/assembler.scm: Write arities into a .guile.arities
  section and associated .guile.arities.strtab.
(system vm debug) can read arity information

* module/system/vm/debug.scm (<arity>): New object, for reading
  arities.  Unlike <arity> in the assembler, this one only holds on to a
  couple of pointers, and doesn't even load in argument names.  Unlike
  the arity lists in (system vm program), it can load in names.  Very
  early days but it does seem to work.
  (find-program-arities, arity-arguments-alist): New higher-level
  interfaces.
Wire up ability to print RTL program arities

* libguile/procprop.c (scm_i_procedure_arity): Allow RTL programs to
  dispatch to scm_i_program_arity.

* libguile/programs.c (scm_i_program_print): Refactor reference to
  write-program.
  (scm_i_rtl_program_minimum_arity): New procedure, dispatches to
  Scheme.
  (scm_i_program_arity): Dispatch to scm_i_rtl_program_minimum_arity if
  appropriate.

* module/system/vm/debug.scm (program-minimum-arity): New export.

* module/system/vm/program.scm (rtl-program-minimum-arity): New internal
  function.
  (program-arguments-alists): New helper, implemented also for RTL
  procedures.
  (write-program): Refactor a bit, and call program-arguments-alists.

* test-suite/tests/rtl.test ("simply procedure arity"): Add tests that
  arities make it all the way to cold ELF and back to warm Guile.
Write docstrings into RTL ELF images

* module/system/vm/assembler.scm (link-docstrs): Write docstrings.
  (link-objects): Link docstrings into the ELF.
procedure-documentation works on RTL procedures

* libguile/procprop.h:
* libguile/procprop.c (scm_procedure_documentation): Move here from
  procs.c, and to make the logic more similar to that of procedure-name,
  which allows RTL programs to dispatch to rtl-program-documentation.

* libguile/programs.c (scm_i_rtl_program_documentation):
* libguile/programs.h:
* module/system/vm/program.scm (rtl-program-documentation): New
  plumbing.

* module/system/vm/debug.scm (find-program-docstring): New interface to
  grovel ELF for a docstring.
procedure-properties for RTL functions

* module/system/vm/assembler.scm (link-procprops, link-objects): Arrange
  to write procedure property links out to a separate section.

* libguile/procprop.c (scm_procedure_properties):
* libguile/programs.h:
* libguile/programs.c (scm_i_rtl_program_properties):
* module/system/vm/debug.scm (find-program-properties): Wire up
  procedure-properties for RTL procedures.  Yeah!  Fistpumps!  :)

* module/system/vm/debug.scm (find-program-debug-info): Return #f if the
  string is "", as it is if we don't have a name.  Perhaps
  elf-symbol-name should return #f in that case...

* test-suite/tests/rtl.test: Add some tests.
fix linker.test

* test-suite/tests/linker.test (link-elf-with-one-main-section): Fix
  test after string table change.
Simplify let-values to let if consumer binds only one variable

* module/language/tree-il/peval.scm (peval): let-values with a consumer
  that has only one argument is the same as let.

* test-suite/tests/peval.test ("partial evaluation"): Add test.
Tree-il post-order rewriter no longer destructive

* module/language/tree-il.scm (pre-post-order): New helper, like
  pre-order! and post-order! but not destructive.
  (post-order): Implement in terms of pre-post-order, and rename from
  post-order!.

* module/ice-9/compile-psyntax.scm (squeeze-tree-il):
* module/language/tree-il/canonicalize.scm (canonicalize):
* module/language/tree-il/fix-letrec.scm (fix-letrec):
* module/language/tree-il/primitives.scm (resolve-primitives): Use
  post-order, and rename from the destructive
  variants (squeeze-tree-il!, canonicalize!, etc).  Adapt callers.

* test-suite/tests/tree-il.test (strip-source): Adapt to post-order.

* test-suite/tests/cse.test:
* test-suite/tests/peval.test:
* module/language/tree-il/optimize.scm: Adapt callers.
Pre-order tree-il rewrites are now non-destructive

* module/language/tree-il.scm (pre-order): Re-implement in terms of
  pre-post-order, and rename from pre-order!.

* module/language/tree-il/primitives.scm (expand-primitives): Adapt to
  pre-order change, and rename from expand-primitives!.

* module/language/tree-il/optimize.scm (optimize): Adapt to
  expand-primitives! change, and rename from optimize!.

* module/language/tree-il/compile-glil.scm:
* module/system/repl/common.scm:
* test-suite/tests/cse.test:
* test-suite/tests/peval.test:
* test-suite/tests/tree-il.test: Adapt to expand-primitives and optimize
  changes.