Carry a let expression's type annotation through to elaboration

[simonspies]

parseLet parsed the annotation in let x : T = e in ... and discarded it, so let b : bool = n in 0 typechecked. AST.Expr.letIn now carries it, and elaboration uses it: with no arguments it is the bound value's own type and lands on the binder, so the bound expression is checked against it; with arguments it is the function's return type and lands on the fix. A destructuring let has no binder to carry it, so an annotation there is now rejected rather than dropped.

Found while writing a test that built a value at an annotated type. Independent of that work, so it is on its own.