add some utf8 decoding convenience functions

[pearcedavis]

Addresses #341.

Let me know if I should add any INLINE pragmas or "since" notes in the docs.

[Lysxia]

Looks good to me. I assume @haskell/text won't object to this.

@pdedmon Can you resolve the conflicts?

[Bodigrim]

decodeUtf8Lenient is fine, but I'm really unsure how much our vocabulary benefits from a synonym and a trivial composition. I do not object however.

[Lysxia]

It's arguable that decodeUtf8' is not a good name (another issue complaining about it), whereas if you see decodeUtf8Either you can better guess what it does. I do share your reservations about also having a Maybe version though...

[pearcedavis]

Shall I reduce this down to just decodeUtf8Lenient then? I don't feel too strongly about the others, though I do agree with the sentiment around decodeUtf8'.

[Lysxia]

Let's do that, yes.

[pearcedavis]

Done and rebased.