Improve docs for 'eval-when'.

* doc/ref/api-macros.texi (Eval When): Explain in detail what each
  condition means, including 'expand' which was previously undocumented.
  Change the example to use (expand load eval) and recommend that set of
  conditions, instead of (compile load eval) which was previously
  recommended and shown in the example.
Improve correctness and consistency of 'eval-when' usage.

* module/ice-9/boot-9.scm:
* module/ice-9/i18n.scm:
* module/ice-9/poll.scm:
* module/ice-9/popen.scm:
* module/ice-9/r6rs-libraries.scm:
* module/oop/goops.scm:
* module/oop/goops/compile.scm:
* module/oop/goops/dispatch.scm:
* module/srfi/srfi-88.scm:
* module/system/foreign.scm:
* module/texinfo/serialize.scm: Change most uses of 'compile' to
  'expand', except where we must avoid it during initial bootstrap
  before the module system is loaded.  Remove redundant uses of
  'compile' where 'expand' is also given.  Standardize on the
  "(expand load eval)" order of conditions.
Fix thread-unsafe lazy initializations.

* libguile/backtrace.c (print_exception_var): New static variable.
  (init_print_exception_var): New static function.
  (scm_print_exception): Remove thread-unsafe lazy initialization.
  Call 'init_print_exception_var' using 'scm_i_pthread_once'.
  Use 'print_exception_var'.

* libguile/continuations.c (call_cc): New static variable.
  (init_call_cc): New static function.
  (scm_i_call_with_current_continuation): Remove thread-unsafe lazy
  initialization.  Call 'init_call_cc' using 'scm_i_pthread_once'.

* libguile/debug.c (local_eval_var): New static variable.
  (init_local_eval_var): New static function.
  (scm_local_eval): Remove lazy initialization using mutexes.
  Call 'init_local_eval_var' using 'scm_i_pthread_once'.
  Use 'scm_variable_ref' instead of 'SCM_VARIABLE_REF'.

* libguile/eval.c (map_var, for_each_var): New static variables.
  (init_map_var, init_for_each_var): New static functions.
  (scm_map, scm_for_each): Remove thread-unsafe lazy initializations.
  Call 'init_map_var' (or 'init_for_each_var') using 'scm_i_pthread_once'.
  Use 'map_var' (or 'for_each_var').

* libguile/frames.c (frame_arguments_var): New static variable.
  (init_frame_arguments_var): New static function.
  (scm_frame_arguments): Remove thread-unsafe lazy initialization.
  Call 'init_frame_arguments_var' using 'scm_i_pthread_once'.
  Use 'frame_arguments_var'.  Use 'scm_variable_ref' instead of
  'SCM_VARIABLE_REF'.

* libguile/goops.c (delayed_compile_var): New static variable.
  (init_delayed_compile_var): New static function.
  (make_dispatch_procedure): Remove thread-unsafe lazy initialization.
  Call 'init_delayed_compile_var' using 'scm_i_pthread_once'.
  Use 'delayed_compile_var'.  Use 'scm_variable_ref' instead of
  'SCM_VARIABLE_REF'.

* libguile/instructions.c (instructions_by_name): New static variable.
  (init_instructions_by_name): New static function.
  (scm_lookup_instruction_by_name): Remove thread-unsafe lazy
  initialization.  Call 'init_instructions_by_name' using
  'scm_i_pthread_once'.

* libguile/ports.c (current_warning_port_var)
  (current_warning_port_once): New static variables.
  (init_current_warning_port_var): New static function.
  (scm_current_warning_port): Remove lazy initialization using mutexes.
  Call 'init_current_warning_port_var' using 'scm_i_pthread_once'.
  Use 'current_warning_port_var'.
  (scm_set_current_warning_port): Remove thread-unsafe lazy initialization.
  Call 'init_current_warning_port_var' using 'scm_i_pthread_once'.
  Use 'current_warning_port_var'.

* libguile/strings.c (null_stringbuf): New static variable.
  (init_null_stringbuf): New static function.
  (scm_i_make_string): Remove thread-unsafe lazy initialization.
  Call 'init_null_stringbuf' using 'scm_i_pthread_once'.

* libguile/strports.c (eval_string_var, k_module): New static variables.
  (init_eval_string_var_and_k_module): New static function.
  (scm_eval_string_in_module): Remove lazy initialization using mutexes.
  Call 'init_eval_string_var_and_k_module' using 'scm_i_pthread_once'.
  Use 'eval_string_var'.

* libguile/throw.c (CACHE_VAR): Remove incorrect macro.
  (catch_var, throw_var, with_throw_handler_var): New static variables.
  (scm_catch, scm_catch_with_pre_unwind_handler): Remove thread-unsafe
  lazy initialization.  Use 'catch_var'.
  (init_with_throw_handler_var): New static function.
  (scm_with_throw_handler): Remove thread-unsafe lazy initialization.
  Call 'init_with_throw_handler_var' using 'scm_i_pthread_once'.
  Use 'with_throw_handler_var'.
  (scm_throw): Remove thread-unsafe lazy initialization.
  Use 'throw_var'.
  (scm_init_throw): Initialize 'catch_var' and 'throw_var'.
Implement SRFI-111 Boxes.

* module/srfi/srfi-111.scm: New file.
* module/Makefile.am (SRFI_SOURCES): Add srfi/srfi-111.scm.
* test-suite/tests/srfi-111.test: New file.
* test-suite/Makefile.am (SCM_TESTS): Add tests/srfi-111.test.
* doc/ref/srfi-modules.texi (SRFI-111): New node.
Minor for-each speedup

* module/ice-9/boot-9.scm (for-each): Minor speedup by unrolling
  tortoise/hare loop.
Add 'positive?' and 'negative?' as primitives.

* module/language/tree-il/primitives.scm (*interesting-primitive-names*)
  (*effect-free-primitives*): Add 'positive?' and 'negative?'.
  (*primitive-expand-table*): Add primitive expanders for 'positive?'
  and 'negative?'.
Primitive expand numerical comparisons with more than 2 arguments.

* module/language/tree-il/primitives.scm (chained-comparison-expander):
  New procedure.
  (*primitive-expand-table*): Add primitive expanders for '<', '>',
  '<=', '>=', and '='.