Blanket impl for types that deref to WordSplitter

[Benjamin-L]

I ran into a situation where it would be useful to use &hyphenation::Standard as a splitter. This PR adds a blanket WordSplitter impl for anything that derefs to a WordSplitter, which should cover cases like this.

[Benjamin-L]

Wow I'm dumb... I accidentally tested without the hyphenation feature. I think this is actually impossible without specialization, so I'm gonna close it.

[mgeisler]

Hey @Benjamin-L,

Okay, no worries! Perhaps we can add some more pointers to the hyphenation feature so it's easier to find?

[Benjamin-L]

That wasn't my problem. I knew about the hyphenation feature, and had been using it. I just forgot to include it when I built this change to test.

[mgeisler]

I see, glad you figured it out in any case :-)