Fix crash in mrb_parser_free() due to freed memory access

yard docs for string.h

Should not define to `Struct` class

reported by https://gist.github.com/miura1729/53fbd8af889c289a79108e38635b2378
fix proposed by @miura1729 in
https://github.com/miura1729/mruby/commit/b1b7933f7aa950cfb747b06327a0d0340f3e4ff8

refactored redundant functions; removed optimized accessors for safety
this fix also close #3178

String#insert should be destructive

Should raise LocalJumpError when no block given

C doesn't have try-catch

Fix warning shift-negative-value

Fix compilation error with GC_PROFILE.

Fix public methods

Update doc, write about add_test_dependency

when multiple mrb_gc_register() were called for the same object

need to find out real memory bug that appears in full-debug/mrbtest

add function for checked mrb_int multiplication

- call __builtin_mul_overflow() if it's available
- perform a 64-bit multiplication for 32-bit mrb_int (default)
- otherwise a much slower method is used

fix MRUBY_VERSION value

improve checked mrb int arithmetic implementation

fix #3152

RUBY_ENGINE_VERSION and MRUBY_VERSION should refer to the same string.

Version checking is not reliable - especially with Clang.
E.g. Apple's Clang (Xcode) uses different version numbers.
A feature check (__has_builtin) is the recommened way.

Add the MRB_HAVE_TYPE_GENERIC_CHECKED_ARITHMETIC_BUILTINS
macro which may be used in other files.