Syntax-location for error in `let` body

[tnelson]

Ideal resolutions of error location for pred vs.let are opposites. Consider this let use from an in-progress model of a kd-tree:

  let d = SearchState.dim | {
    all ch: n.left.*(left+right)  | ch.key[d] < n.key[d]
  ...

If the SearchState.dim expression is ill-typed, a join error is produced. Ideally the error would be reported at the location of the declaration body, not the use site.

However, pred resolution is the opposite: we want to report error location at the use site, not the declaration.

Currently, Forge uses pred style resolution for everything. That was the right call for the majority of errors, but causes confusion for let.