`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]

This issue is a correction error at 5a682bfc.

The `${build_dir}/mrbgems/gem_init.c` generation condition has been changed
at #5010, but it is somewhat insufficient, for example, there was a problem
with `rake && rake test`, which was also regenerated in `rake test`.

Fix `ssize_t` for mingw; ref #5030

Legacy MinGW and MinGW-w64 had own `ssize_t`.

Remove the prototype declaration `mrb_free_backtrace()`

This function is removed by 9644ad51.

Fix compiling mruby-io on msc with UTF8 enabled