Add more *_type functions

[fingolfin]

We have

• abs_series_type
• dense_matrix_type
• dense_poly_type (but no sparse_poly_type or poly_type exists?)
• elem_type
• mpoly_type (presumably this is always sparse)
• parent_type

Hecke adds a few more, many specific to certain types, but perhaps these could be useful for more things and should be defined here.

• base_field_type
• base_ring_type
• base_type
• coefficient_type
• ideal_type
• matrix_algebra_type

More that come to mind (to be extended)

• matrix_space_type (if we had this, we could probably use a single MatrixSpace / matrix_space method for all the various implementation, as their constructors all take the same arguments)
  • this is redundant if we merge PR Make Generic.MatSpace truly generic #1318
• ...