allocate `history_path` dynamically

`PATH_MAX` doesn't have to be defined and if it is, then its value is
usually incorrect (possible buffer overflow).
Use `malloc` and `snprintf` instead to prevent this.

https://www.gnu.org/software/hurd/community/gsoc/project_ideas/maxpath.html
call `mrb_malloc_simple` instead of `malloc`
Add field `allocf_ud` to replace current `ud`.
Since some use it as `mrb_state` associated user data.
add Module#module_function
check if block is given in eval_under(); fix #2486
Fix memory leak for test driver
Merge pull request #2488 from ksss/leak-test

Fix memory leak for test driver
Silence "unused parameter" warnings in C++

This has been done already, but the fixes were accidentally
removed in 249f05e7d70761823ef07a990276f9200e8b3302.
Merge pull request #2489 from Ancurio/silence_cpp_warn

Silence "unused parameter" warnings in C++
Merge pull request #2477 from take-cheeze/allocf_ud

Add field `allocf_ud` to replace current `ud`.
Merge pull request #2478 from dreamedge/add_module_function

add Module#module_function
Refactor sym_equal() in symbol.c
Merge pull request #2490 from suzukaze/refactor_symbol

Refactor sym_equal() in symbol.c
Merge pull request #2462 from cremno/mirb-malloc-history_path

mirb: allocate `history_path` dynamically
remove unnecessary CHAR_BIT != 8 check

mruby uses uint8_t which implies CHAR_BIT == 8.
Use Boolean macro in variable.c
Merge pull request #2494 from suzukaze/refactor-variable.c

Use Boolean macro in variable.c
fix typo with MRB_INT16; fix #2495
Time#to_i and Time#usec should care about mrb_int overflow on MRB_INT16; ref #2495
add MRB_API modifiers to mruby API functions