maint: reflect recent file renames in README.

* README: Version dependencies are now in bin/zmacs.
Reported by Reuben Thomas.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
tests: use zmacs wrapper script to set paths correctly.

* tests/run-lisp-tests.lua (zile_cmd): Use lib/zmacs/zmacs path-
setting bin/zmacs wrapper.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
zmacs: simplify variable get/set interfaces.

Mantain a consistent abstraction between internally tabulated
"_" variables and zlisp "-" delimited equivalents, by way of
an internal API.
* lib/zile/variables.lua: Remove.
* lib/zile/zile.mk (dist_pkgdata_DATA): Remove
lib/zile/variables.lua.
* lib/zmacs/eval.lua (varname_map): Remove.
(main_vars): Rewrite as a proxy table manager to automatically
convert between zlisp "-" delimited and zile "_" delimited
variable names.
(Defvar, est_variable_buffer_local, get_variable)
(get_variable_bool, set_variable): Rewrite accordingly.
(get_variable_doc): New function, return variable doc-strings.
* lib/zmacs/zlisp/help.zl (describe-variable): Use
get_variable_doc() to abstract access to variable metadata.
* lib/zile/buffer.lua (init_buffer): Move from here...
* lib/zmacs/eval.lua (init_buffer): ...to here, and add buffer-
local variable table.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>