Related to #3662

When MRB_INT64, valid value range of mrb_int is bigger than double,
which only has 53 bits significant precision.

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