Add thenValidate combinator

[jecaro]

This is a little combinator I found convenient when using the library.

It allows you to apply a validation function to the deserialized value. It is thus useful to enforce invariants on custom data types, for example, check that a Int is strictly positive etc...

[tomjaguarpaw]

Hi @jecaro, thanks for this. Just to set expectations, tomland (and pretty much the whole Kowainik ecosystem) is in something like "emergency maintenance only mode" right now. I maintain it enough to keep things building with new GHC releases, but that's it. @vrom911 is the one with sufficient authority to determine changes to the package, and she has not been active recently.

[jecaro]

Sure, I understand the situation.

In any case, having this little piece of code around, even in an unmerged PR, could be useful to people running into the same needs.

Thanks for keeping those libraries up to date with recent GHC versions 🙏