Add prisms for working with prefixed and suffixed text

[Taneb]

Closes #981

[RyanGlScott]

@Taneb, is this something you're planning to work on soon? I'm interested in landing this before the next lens release. If you don't have time, then no worries—I can pick it up (assuming your branch gives the appropriate permissions to lens contributors).

[Taneb]

I'd forgotten about it, being honest. My main issue was I couldn't figure out how to run the doctests locally, so I'd have had to do a lot of noisy trial and error to get things passing. Other than that I'm happy to work on it. I think the branch is on this repo so anyone with permission for ekmett/lens should be able to push to it

[RyanGlScott]

My apologies, I didn't realize you were blocked. Here is my workflow for running the doctests:

1. Build the library with cabal build.
2. Download cabal-docspec from https://github.com/phadej/cabal-extras/releases/download/cabal-docspec-0.0.0.20210111/cabal-docspec-0.0.0.20210111.xz
3. Run cabal-docspec in the top-level lens checkout directory. (Make sure the same GHC version you used in step (1) is on your PATH.)

If that doesn't work, feel free to shout.

[RyanGlScott]

I pushed a commit which fixes the build errors on old GHCs. Or so I thought, at least. There's a separate issue on GHC 7.8.4:

[29 of 86] Compiling Control.Lens.Prism ( src/Control/Lens/Prism.hs, /__w/lens/lens/dist-newstyle/build/x86_64-linux/ghc-7.8.4/lens-5.1/build/Control/Lens/Prism.o )

src/Control/Lens/Prism.hs:397:31:
    Not in scope: ‘BS.stripPrefix’
    Perhaps you meant one of these:
      ‘TS.stripPrefix’ (imported from Data.Text),
      ‘TL.stripPrefix’ (imported from Data.Text.Lazy),
      ‘TS.stripSuffix’ (imported from Data.Text)

src/Control/Lens/Prism.hs:401:31:
    Not in scope: ‘BL.stripPrefix’
    Perhaps you meant one of these:
      ‘TL.stripPrefix’ (imported from Data.Text.Lazy),
      ‘TS.stripPrefix’ (imported from Data.Text),
      ‘TL.stripSuffix’ (imported from Data.Text.Lazy)

src/Control/Lens/Prism.hs:431:33:
    Not in scope: ‘BS.stripSuffix’
    Perhaps you meant one of these:
      ‘TS.stripSuffix’ (imported from Data.Text),
      ‘TL.stripSuffix’ (imported from Data.Text.Lazy),
      ‘List.stripSuffix’ (imported from Data.List.Lens)

src/Control/Lens/Prism.hs:435:33:
    Not in scope: ‘BL.stripSuffix’
    Perhaps you meant one of these:
      ‘TL.stripSuffix’ (imported from Data.Text.Lazy),
      ‘TS.stripSuffix’ (imported from Data.Text),
      ‘List.stripSuffix’ (imported from Data.List.Lens)

stripPrefix was introduced in bytestring-0.10.8.0, and unfortunately, GHC 7.8.4 comes bundled with an older version of bytestring. This doesn't seem easy to work around unless we backport the entire implementation of stripSuffix, which doesn't sound fun. Perhaps we should land #977 first and rebase on top of that.

[RyanGlScott]

The CI finally passes. Hooray!

[RyanGlScott]

I've pushed some commits which add more Haddocks, more changelog descriptions, and makes things more backwards-compatible. I think that takes care of the remaining review suggestions, so unless anyone notices something I've overlooked, I'll plan to merge this within a day or so.