We already did this, but missed a few cases due to the way arguments are
elaborated. So now, when checking an LHS, we don't allow LHS argument
types to be inferred from the pattern, but rather they must be inferred
from elsewhere. To do this, we keep track of the constraints which would
be solved when inferring the type, and make sure they don't solve any
new metavariables. Fixes#1510, and also now gets the error location
right as a bonus!
We stored them as equations between terms, I think because terms are
easy to re-evaluate with new information, and because I thought we might
want to save them out. It's not usually a problem to do that. However...
Going back and forth between terms and values can be expensive if
we're stuck in the middle of a complicated unification problem, the like
of which can turn up a lot if your types are complicated. So, we need to
be able to handle this.
Now store the postponed problems as NF, rather than Term, and add a
fuction to resume evaluating a NF with an updated context.
helps with readability since these, especially named-IPi, come up a lot
didn't change everything that could need it like PiInfo or BindMode
PiInfo rarely has DefImplicit (so far) and BindMode hasn't come up a lot (so far)
reduced indentation for TTImp Show implementation
