Like in delayed ambiguity resolution, we need to reevaluate the target
type because it might have changed - and that's why we delayed in the
first place!
This has shown up a problem with 'case' which is hard to fix - since it
works by generating a function with the appropriate type, it's hard to
ensure that let bindings computational behaviour is propagated while
maintaining appropriate dependencies between arguments and keeping the
let so that it only evaluates once. So, I've disabled the computational
behaviour of 'let' inside case blocks. I hope this isn't a big
inconvenience (there are workarounds if it's ever needed, anyway).
Also fix a bug where the elaborator state wasn't updated on completing
the delayed elaborator, which could cause issues with implicitly bound
names in particular.
Need to run delayed elaborators before binding implicits, since there
might be some inside the delayed elaborator. Also reorganise TTC
implementations so they're all in one place.
