Extract memsize to gem

Those methods are originally CRuby specific.
Co-authored-by: Yukihiro "Matz" Matsumoto <matz@ruby.or.jp>

Those tests succeeds only on some configuration.

The old code compiles only on `MRB_NO_BOXING`.

`mrb_value` may or may not be struct according to configuration.

`C` retrieves a `mrb_value` that refers a class/module.
`c` retrieves a `struct RClass*` pointer to a class/module.

`ObjectSpace.memsize_of_all` takes a class and count memory size of all
instances of the class and its subclasses (if any).

Add ObjectSpace.memsize_of_all

This bug has been there since mruby 1.4.0 (2018-04).

Also avoid `mrb_funcall` to minimize VM recursion.

Memory size of a Fiber is calculated by stack size only in CRuby.

This is enhancement from CRuby's `memsize_of`. We need to change the
CRuby first for the enhancement.

Fixed shift width for `MRB_ENV_SET_BIDX()`

Add ObjectSpace.memsize_of

ref c07f24cd and close #5035

Fix `${build_dir}/mrbgems/gem_init.c` generation condition; ref #5010

Set `STDOUT.sync=true` when testing on AppVeyor [skip travis]