Add non-throwing indexMaybe function (continues #146)

[cole-miller]

This is a continuation of @FintanH's work in #146. I've rebased his commits onto the current master.

[FintanH]

Thanks for bringing it to the finish line ❤️

[Bodigrim]

W. r. t. 02c54aa - please retain (!?). It is used as an operator for safe lookups in many packages: containers, vector, massiv, ral, relude, just to name a few. It is actually indexMaybe which is not featured by anyone else (except text-short).

[cole-miller]

I think I've implemented most of the outstanding comments from the previous review; the things I'm still unclear about are:

• Is there a preferred alternative to this definition?

indexMaybe = (fmap w2c .) . B.indexMaybe

• Should ($) be strict in all versions of the indexMaybe definition, or only in those for the "strict" modules? (This is probably a dumb question, sorry.)

[Bodigrim]

Is there a preferred alternative to this definition?

Let it be as is, it is potentially marginally better for inlining.

Should ($) be strict in all versions [...]?

Yes, make it strict in all versions.

[cole-miller]

Thanks @Bodigrim @fumieval @sjakobi for your guidance, and @FintanH for doing most of the work :)