Support deserialising Word (and Word8 etc.) from PersistDouble

[isomorpheme]

This implements approach no. 2 as discussed in #831. Just after authoring this I noticed that there's also PR #1096 which seems to do something more like approach no. 3, and it would conflict with this PR (e.g. it's stricter about deserialising PersistDoubles), though it looks like it's stalled for now.

Closes: #831

Before submitting your PR, check that you've:

• Added @since declarations to the Haddock I'd like to add these to the relevant instances documenting the change in behaviour, but it looks like Haddock doesn't support adding a comment or the like.
• Ran stylish-haskell on any changed files.
• Adhered to the code style (see the .editorconfig file for details)

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Bumped the version number if there isn't an (unreleased) on the Changelog
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)