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.

Remove needless assignment

d4ee409a should remove this line.

Use stack directly

See https://github.com/mruby/mruby/pull/3142#issuecomment-201138873

MRB_INT_MAX does not have corresponding positive integer

Avoid Syntax Error on CRuby v1.8

CRuby v1.8 dose not support named capture

fix syntax highlighting

`a` in `a.m += c` or `a[b] += c` should be evaluated only once.

Add missing regs update

mrb_vm_define_class() may realloc() mrb->c->stack because it calls
mrb_funcall() for inherited hook. If mrb->c->stack is realloc()-ed, regs
refers orphan address.