Find a location for Pattern_White_Space

[erickt]

rustc_unicode recently added support for Pattern_White_Space in rust-lang/rust@24578e0. Syntex needs this support somewhere, and @kwantam mentioned that unicode-xid might be the right place for this functionality.

cc @SimonSapin

[kwantam]

@erickt thanks for raising this issue!

@SimonSapin my vote would be to insert this table in xid for now, and then when we eventually get around to making an ur--Unicode crate with all the tables, we can relocate it there.

The reason to put it in xid is that, roughly speaking, xid and pattern_white_space predicates are often both used in parsing, thus xid is a better match than any of the other existing crates; and I don't think it's necessary or productive to create an entirely new crate for pattern_white_space.

Thoughts?