Make Coq Ident type into a newtype

[brianhuffman]

Making the Coq Ident type into a type that is distinct from String would help to avoid the kind of bugs that aba230b68bf06700e845d00f452e5c7230342483 is trying to fix, where in the translator code we simply forgot to translate some of the bound names. (See comments on GaloisInc/saw-core-coq#29.)

Another advantage of having an abstract type is that we could freely change its representation later (e.g. to Text). It would also make the code arguably easier to understand and to maintain.