Implement onleave/$onleave

New, experimental feature that allows commands to run
when a proc or interpreter exits.

If the $onleave variables exists at proc or interp exit,
it is treated as a list of scripts to evaluate.

The [onleave] command is a helper to add scripts to $onleave.

See tests/onleave.test

Signed-off-by: Steve Bennett <steveb@workware.net.au>