Without this cast, `long int:63` can cause warnings.

Redundant `mark_context() is far better than crashes.

The fix was proposed by @block8437.  Thank you.

This reverts commit e26e1186.
Fix #3664

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