Sorry for the less than informative title :). Lots going on here. It
started as an attempt to fix unification to deal with laziness coercions
in trickier places, but unearthed a couple of tricky and interconnected
issues that are hard to unpick into a single patch. So, this fixes a few
things:
- default hints should only be resolved on the current elaboration (e.g.
nested function definitions, not the outer definition which might not
yet be complete)
- delayed elaborators should be allowed to have nested delayed
elaborators, which means disambiguation is a little bit better
- we should delay elaborating arguments where the type isn't known yet,
because later arguments may resolve the type, and we can use this to
help with disambiguation/laziness coercions
- other bits and pieces arising
