Support for tuples

[SorteKanin]

I would like to be able to make a set of tuples, like so:

use phf::phf_set;
static MY_SET: phf::Set<(&str, u32, &[&str])> = phf_set! {
    ("some value", 1, &["even", "more", "values"]),
};

This is currently not possible it seems. I am guessing it's also not possible to use tuples as keys in phf::Map

You can work around this by using phf_map with the first value as key and the rest as the resulting value but that isn't very ergonomic and also only works if the first value is unique.

[stephanemagnenat]

As with #183 it would be good for this limitation to be clearly documented.