Possible workaround for #2824 (MRI GC bug)

This is a moderately ugly workaround for the MRI garbage collection
bug (see the ticket for details).

I explored several other potential solutions (notably, monkey
patching the routines that trigger the bug) but none of them were
satisfactory. Monkey patching sub, gsub, sub!, gsub!, etc., for
example, either changes the scoping of $~, $1, etc. in a way that
could potentially subtly change the meaning of programs or (if you
are clever) faithfully reproduces the behaviour of MRI--including
the memory leak.

I decided to go with the standardized and somewhat obnoxious never-
used optional argument as it was easy to automatically insert and
should be even easier to automatically find and remove if a better
fix is developed. It also should be obtrusive enough to escape
accidental removal in refactoring.