Add API `mrb_atexit()`.
`mrb_final_mrbgems` will be called as mrb_state atexit function.
Maybe useful in #1844.
Fix invalid array initialize for VS2012
Merge pull request #2329 from bggd/patch-4

Fix invalid array initialize for VS2012
Replace TLSF-2.4.6 (GPLed) to tlsf-3.0 (Public domain).
Support string key in `Struct#[]=`.
Add invalid key type check in `Struct#[]=`.
Merge pull request #2330 from take-cheeze/struct_set_str

Improve `Struct#[]=`.
Add support for baisoku.org's TLSF.
Avoid crashing in "stack extend" test.
Comment out for now. This cannot be passed in this tree.
Merge branch 'feature/change-to-baisoku-tlsf-allocator' into develop
Publish MRB_STACK_GROWTH and MRB_STACK_MAX.
Add MrbState module and stack_limit, stack_limit= method.
Merge branch 'feature/add-mrb_state-module-and-stack_limit-settings' into develop
Add stack_size getter method. Add allocate_stack method.
Merge branch 'develop' into feature/toppers-jsp-bfin
Comment MRB_USE_TLSF macro definition out.

This is for host environment. Define it in build_config.rb.
Add function declarations for monami-ya.mrb.
Add MRB_DISABLE_HOSTED macro.

This is for freestanding C/C++ environment.
Not only stdio but also all hosted functions should be disabled when it is defined.
Add MRB_DISABLE_HOSTED around malloc().