librumur: support for enum types

The implementation of these departs somewhat from CMurphi. Where CMurphi
implements each member of each enum as a separate value (i.e. enum members in
distinct enums will still have different numerical representations), we simply
implement enums as C would. We'll need to do a little more work to prevent
accidental comparison of distinct enum values, but this representation should
allow us to more efficiently encode enum values in the state. This only matters
for input models that contain large enum types, but empirically it seems like
this is much more common than one might think. E.g. see [0].

The EnumValue class and related symbol table declaration implemented in this
commit is a bit awkward. It would be nice to implement this in a cleaner way,
but I couldn't immediately think of a better alternative.

  [0]: https://bitbucket.org/jderick/preach/commits/e7cecfc142253799bdf5d5cc5ed60015ec5cd0a8