new_frontend
For example, `mkFreshExprMVar none MetavarKind.synthetic` should create a fresh synthetic metavariable `?m` with type `?t` where `?t` is a fresh natural metavariable. If users want a synthetic metavariable `?t`, then it must create it themselves.
Two of them are still broken due to a bug in the new elaborator.
ensureHasType