a few Either functions

[anovstrup]

This PR adds Either.fold, Either.left, Either.right, and Either.unwrap functions with associated documentation. These definitions also appear in #57 (not yet reviewed/merged), but they did not include documentation in that PR. Tests are omitted, since the definitions are very simple and obviously correct.

Code review

The changes summarized below are available for you to review, using the following command:

pull-request.load https://github.com/unisonweb/base:.trunk https://github.com/anovstrup/unisonweb-base:.prs._either1

Added definitions:

 Either.fold       : ∀ o 𝕖 i1 i. (i1 ->{𝕖} o) -> (i ->{𝕖} o) -> Either i1 i ->{𝕖} o (+3 metadata)
 Either.fold.doc   : Doc (+2 metadata)
 Either.left       : Either l r -> Optional l (+3 metadata)
 Either.left.doc   : Doc (+2 metadata)
 Either.right      : Either l r -> Optional r (+3 metadata)
 Either.right.doc  : Doc (+2 metadata)
 Either.unwrap     : Either a a -> a (+3 metadata)
 Either.unwrap.doc : Doc (+2 metadata)

[runarorama]

Merged in base hash #o6r70946gi. Thanks! 🌈⭐