This PR adds a procedure that efficiently transforms `let` expressions
into `have` expressions (`Meta.letToHave`). This is exposed as the
`let_to_have` tactic.
It uses the `withTrackingZetaDelta` technique: the expression is
typechecked, and any `let` variables that don't enter the zeta delta set
are nondependent. The procedure uses a number of heuristics to limit the
amount of typechecking performed. For example, it is ok to skip
subexpressions that do not contain fvars, mvars, or `let`s.
