tfsat.c: Handle cases of defined types appearing in embeddings
Add multivariate polynomials into the algebra library.

They've been sitting around for a while, and it's probably time they
got committed.  Note: I've done no testing beyond 'does it compile'

One small issue: there's a 'combine' method in alg_ezgcd which does not
have a definition, only a specification (I guess it was removed from the
original sources at some point).  This means multivariate gcd won't work
as it stands.