Split Yoneda embedding into separate module and rename helper functions

[dancewithheart]

Split the Yoneda embedding definitions out of
Cubical.Categories.Yoneda into a separate module
Cubical.Categories.Yoneda.Embedding.

The main names are re-exported from Cubical.Categories.Yoneda
to preserve the existing interface.

Also rename helper functions:

• yo-yo-yo → yonedaToElement
• no-no-no → elementToYoneda

Partially address #1285.

[maxsnew]

What's the impetus for splitting it into two files?

[dancewithheart]

I treated the split as part of the cleanup: the file mixes the core Yoneda lemma
(yoneda, yonedaᴾ, yoneda*, yonedaᴾ*) with the embedding/application
definitions (yo, YO, isFullYO, isFaithfulYO, isFullyFaithfulYO).

The idea was simply to separate those. I re-exported from Yoneda to preserve
the current interface.

If the extra file is not desirable, I'm happy to keep everything in one file
and limit the PR to the renames.

I also thought it might be useful to import the lemma without the embedding
machinery, though that alone may not justify a separate file.