Extend `tropical_semiring_map` to allow maps from rings

Required in preparation of working with Puiseux polynomials, will require some rethinking of how `tropical_semiring_map` is currently used:

Currently: If `tropical_semiring_map` can be constructed then tropical GB can be computed (and things that rely on it, e.g. tropical varieties).

Future: Just because `tropical_semiring_map` can be constructed does not mean tropical GB can be computed.