gh-38487: convenience methods for field embeddings
Currently, the best(?) way to compute embeddings between fields is
somewhat hidden inside the `Hom` object. In this patch we add helper
methods to access such embeddings. In addition, we add an
`.algebraic_closure()` method to non-prime finite fields.
These seem useful.
URL: https://github.com/sagemath/sage/pull/38487
Reported by: Lorenz Panny
Reviewer(s): Giacomo Pope, Sebastian A. Spindler
