Put Expr type checking under a flag.
Put Expr type checking under a flag.

This lets us query the types of expressions without worrying about linearity or
typeclass constraints. It also makes the interpreter (which uses `getType` a
lot) quite a bit faster.

google-research/dex-lang,google-research/dex-lang,google-research/dex-lang,google-research/dex-lang,google-research/dex-lang,google-research/dex-lang