Replace String with Symbol

This branch accompanies https://github.com/ucsd-progsys/liquid-fixpoint/pull/39, replacing `String`s with `Symbol`s (`Text`s internally). There are a few rough spots remaining where `String`s show up, mostly having to do with interop with GHC. It looks like we might be able to transform `Text -> ByteString -> GHC.FastString -> GHC.Name` but I haven't tried it yet. Another area where `String`s show up is the whole `Annotation` business, but that's far off the critical path so I'm not very concerned about it.