Use `IntMap` to implement `TypeEnv` more efficiently

[brianhuffman]

PR #884 improved evaluation performance in the Cryptol REPL by using Data.IntMap instead of Data.Map to keep track of the value variables in the evaluation environment. A similar performance improvement is likely possible by switching the type variable portion of the environment (TypeEnv) to use IntMap as well.

cryptol/src/Cryptol/Eval/Type.hs

Line 94 in 0000ffb

type TypeEnv = Map.Map TVar (Either Nat' TValue)

One question is whether this can be done with a single IntMap, or will it require two maps (one for TVFree variables, and one for TVBound variables). Each of those constructors contains an Int as a unique ID, but I'm not sure if they are guaranteed to be disjoint from each other. Or maybe we need separate maps for another reason.