Another quick fix from jmuizelaar for that "some ocaml versions dont't notice wrong wildcard count in patterns" bug.

Tidy up handling of unimplemented features. These are expected (if undesirable) sorts of error, we should handle better than "backtrace and exit 2".

Add a test for an obvious-seeming (but not actually legal) kind of cast attempted in issue #115, downgrade bug to an err in type.ml so you get a better message.

Fix IL translation of pattern-alt to allow a value of mutable/constrained type to be the subject of a pattern-alt.

Desugar the head stmts all the way out of STMT_for, rather than stashing them in the node. That's only necessary for STMT_while.

Fox the clauses that actually satisfy box and mutable constraints; in all the confusion I had got those wrong.

Remove TYSPEC_loggable. Everything's loggable (or will be, eventually; the backend can catch unimplemented cases until then).

Remove the catch-all in type.ml, add cases for every stmt (a couple more handled; mostly still stubs).

Rename 'exterior' to 'box' and 'interior' to 'local' (at least wrt. slots; keep MEM_interior for describing interior-parts-of-allocations)

Add two move variants of "acceptable mismatches to recover from" when the mut_ok or ext_ok flags are set.