Rename dense_poly_type and/or mpoly_type (and their *_ring_type counterparts)

[fingolfin]

Right now we have

• dense_poly_type and mpoly_type

During our big renaming sessions, we agreed that "dense" would not be specified (at least in types), but sparse would be. Also there is univariate versus multivariate... So we might want to rename these types to be more consistent. Here are some suggestions:

1. dense_poly_type and mpoly_type (leave it as it is)
2. poly_type and mpoly_type (drop the dense)
3. poly_type and sparse_poly_type (drop dense, add sparse)
4. upoly_type and mpoly_type (emphasis univariate vs. multivariate)
5. univariate_poly_type and multivariate_poly_type

Personally I favor option 2.

CC @thofma @fieker @mgkurtz