get_filename_table_size realloc fix.

Updated the documentation to better match the way things work now that 172cd281 is merged.

Automatically detect toolchain if it is visualcpp, and hence we drop vs2010.rake altogether - it’s no longer needed.

build with MRB_GC_FIXED_ARENA on travis

Automatically detect toolchain if it is visualcpp, and hence we drop vs2010.rake altogether - it’s no longer needed.

You will not see "arena overflow" error anymore, but I encourage gem authors
to check your gems with MRB_GC_FIXED_ARENA to avoid memory broat.

declare struct mrb_irep.

Fixed so that it builds with VS2013. VLA:s are not supported, because of its strict C89 world view.

renamed mrb_exc_new3 to mrb_exc_new_str

codegen_malloc/realloc: static + simple mem alloc

GCC: add -std=gnu99 to CFLAGS

- this change prevents issues like #1539 in the future
- Clang's default is already gnu99 but GCC's is gnu89
- mruby is C99 (+ GNU exts), not C89 (+ GNU exts)

- more descriptive name (there's no mrb_exc_new2)
- no wrapper (macro/static inline function) for
backwards compatibility because nothing besides
mruby itself seems to be calling this function:
<http://mruby-code-search.ongaeshi.me/home?query=mrb_exc_new3>

- there's no reason for both to have external linkage
- use simple versions of memory allocation functions
which are returning NULL instead of longjmp-ing,
so codegen_error will be called to cleanup.

irep->pool[n].type should be IREP_TT_*, not MRB_TT_*

fix warning: "implicit conversion from enumeration type 'enum mrb_vtype' to different enumeration type 'enum irep_pool_type' [-Wenum-conversion]"