Instead of preserving a backtrace in `mrb_state`, `mrb_exc_set`
keeps packed backtrace in an exception object. `#backtrace` unpacks
it to an array of strings.

String#concat: Try to convert when not string

Avoid call to_s during backtrace traversal

Make string embad from shared

Change return back to break in the default case.

Fix segmentation fault ref: #3648

Adjust to the optimum type

Sprintf refactor

Raise argument error with the better message, at the only place
where the size to fill can be negative.

Instead of loop or `memcpy` after `CHECK`, use dedicated macros,
`FILL` and `PUSH`, respectively.

Add macOS support to mrb_ro_data_p

This adds macOS support to mrb_ro_data_p, fixes #3636

Restore 32-bit OSX builds

ref #3492 #3515 #3517

This reverts commit d2cad9ac.

To avoid redundant `mark_context()`.

This reverts commit a1d32af9.

Fix space flag with inf/nan