Fix wrong the number of marked objects of MRB_TT_ENV in gray mark phase

If MRB_TT_ENV has stack shared flag or bidx flag, flags is too large
than the real stack size.

Fix array replace leak error in mruby-uri.

sort method should not consume system stack.

Fix undefined error of pointer dereference.

Avoid using `mrb_yield` in C code. The function is not recommended.
Because it doesn't work well with fibers.

`String#lines` (with a block) is now implemented in Ruby.

Otherwise the duplicated object may have shared entities that
should not be modified in-line.

When arguments `copy` and `orig` are the same object.

Fallback to float when compiled binary with 64bit compiler.

closes #3997.

The bug and the fix were reported by https://hackerone.com/pnoltof

When object allocation rate during incremental GC is too high, the
`gc->majorgc_old_threshold` becomes too big. This means major GC
start slower and old objects stay longer (and consume too much memory).

Because 200% is consumes too much memory for some cases.

Because unlike other GC ratio constants, it's not a default value.

Those barriers are inserted to fix #3699 but all living fibers are
marked from `mark_context()` anyway now.

Fix build error on Windows with MRB_BUILD_AS_DLL

Error message:

    C:\projects\groonga\vendor\mruby-source\include\mruby.h(1098): error C2375: 'mrb_gc_arena_save' : redefinition; different linkage [C:\projects\groonga\vendor\mruby\mruby.vcxproj]
              C:\projects\groonga\vendor\mruby-source\include\mruby.h(1083) : see declaration of 'mrb_gc_arena_save'
    C:\projects\groonga\vendor\mruby-source\include\mruby.h(1099): error C2375: 'mrb_gc_arena_restore' : redefinition; different linkage [C:\projects\groonga\vendor\mruby\mruby.vcxproj]
              C:\projects\groonga\vendor\mruby-source\include\mruby.h(1084) : see declaration of 'mrb_gc_arena_restore'

Fix possible heap use after free in `mrb_exec_irep` and stack expanding.

Fixed memory failure on mruby debugger.