It turns out the codec cache and the refleak hunting mechanism in regrtest *really* don't like each other if you use closures to implement your test codecs :)

Anyway, the attached patch tweaks the tests to handle refleak hunting (and running the refleak hunter indicates there aren't any leaks). I'll be committing this one immediately after posting this.

And like MAL, I'm keen to stick with a fully private API at this point, as I believe we need to get the status quo clearly documented before we can start talking about making changes to the public APIs.

I think both this issue and issue 7475 show that until we have a common understanding amongst the core development team of what's possible now, it's hard to discuss:

- what's currently possible that we would like to deprecate? (opinions differ strongly on this one - MAL and I think the status quo is mostly OK, but could use some better type inference support, Victor would like to see support for anything beyond text encodings, binary transforms and text transforms deprecated)
- what's not possible that we would like to make possible? (e.g. removing cached codecs, removing search functions, reordering search functions, type inference through the codec machinery)
- what's already possible that we would like to make easier? (e.g. registering new codecs)