Fixes for quantifiers + let-such-that expressions

[jamesbornholt]

Fix two bugs involving combining let-such-thats with quantifiers:

• When inlining lets during trigger selection, we need to check the let is exact (i.e., not a let-such that)
• After fixing that issue, a let-such-that within a quantifier generates invalid Boogie, because let desugaring reuses a (mutable) substitution map that has had the quantified variables removed