Record and check initial implementation

Record and check initial implementation

Really don't like that we use hard-coded addresses.  There are examples
in the existing testsuite that use options of hard-coded addresses, but
I want something more general that we can actually test the things we
need to test with.

Here we add an initial implementation to do such a thing.

This initial implementation has quite a lot of problems, but it adds
a lot in the fact that we can write testcases which should work across
different setups.
Hopefully we can work out the problems with use (or maybe identify that
the problems don't actually matter very much in practice) and eventually
upstream something better.

To document the problems:
  - The implementation means that recording something actually puts that
    into the regexp_diff namespace which could shadow existing variables.
  - We don't have a way to say "the previous value", but always have to
    write some TCL procedure to return that previous value.