Top-level local variables contained junk after ab25eaea

The old order generated some strings with their class set to NULL.

Even though the returning value is retrieved from the stack,
it may be freed if GC is caused during stack rewinding
(e.g. ensure calls).

They didn't work well with inexact numbers (inf,nan). Fix #3671

Fix #3668

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.