Fails to lex underscore in unicode escapes

[dtolnay]

This test case from the compiler test suite results in a lex error:

https://github.com/rust-lang/rust/blob/1.22.1/src/test/run-pass/issue-43692.rs

This was implemented upstream in rust-lang/rust#43716 and stabilized in Rust 1.22.