Add ObjectSpace.memsize_of

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

Update Windows CI settings

object_id of true, false, and undef are all 0

Should not free the pointer in `realloc` since it can cause
use-after-free problem.

Work around MSC optimization generating non functional code with Array#shuffle