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.

Refactoring for `Enumerable#each_with_object`

`each_with_object` arity should be 1

Reimplement `Enumerable#zip` with Enumerator

Support block yielding for Enumerable#zip

for fix some specs
- [passes each element of the result array to a block and return nil if a block is given](https://github.com/ruby/spec/blob/a585ec35d185435e5c11f371ba4ed2a29d8817bd/core/enumerable/zip_spec.rb#L11-L17)
- [converts arguments to arrays using #to_ary](https://github.com/ruby/spec/blob/a585ec35d185435e5c11f371ba4ed2a29d8817bd/core/enumerable/zip_spec.rb#L23-L27)
- [converts arguments to enums using #to_enum](https://github.com/ruby/spec/blob/a585ec35d185435e5c11f371ba4ed2a29d8817bd/core/enumerable/zip_spec.rb#L29-L34)
- [gathers whole arrays as elements when each yields multiple](https://github.com/ruby/spec/blob/a585ec35d185435e5c11f371ba4ed2a29d8817bd/core/enumerable/zip_spec.rb#L36-L39)

Revert #3984 .

Catch up with latest mruby-socket.

Return nil if type differ in `String#<=>`.

Add `:path` option for git repository.