Add more aggressive defaulting rules for REPL expressions.
Add more aggressive defaulting rules for REPL expressions.

If we encounter a type that is not constrained by `Literal` or `FLiteral`
constraint, we will try to default such types to `Integer`, then
`Rational`, then `Bit`, in that order.

Fixes #877

GaloisInc/cryptol,GaloisInc/cryptol,GaloisInc/cryptol