First attempt at recursion in GV, with translation to CP.

In addition to 'let rec', I've added a more low-level GV term 'corec',
corresponding directly to the corecursion combinator in CP.  This
simplifies the translation of 'let rec' into CP.  However, I do not yet
have an implementation of 'corec' in the (asynchronous) semantics of GV.

'let rec' requires a fair bit of type annotation; I'm not sure it is all
necessary.
Tweak Peano example.