Also, small formatting fix and removal of debugging test.

Previously, we had some inconsistencies in how we were handling ASCII
word boundaries. In particular, the translator was accepting a negated
ASCII word boundary even if the caller didn't disable the UTF-8 invariant.
This is wrong, since a negated ASCII word boundary can match between any
two arbitrary bytes. However, fixing this is a breaking change, so for
now we document the bug. We plan to fix it with regex 1.0. See rust-lang#457.

Additionally, we were incorrectly declaring that an ASCII word boundary
matched invalid UTF-8 via the Hir::is_always_utf8 property. An ASCII word
boundary must always match an ASCII byte on one side, which implies a
valid UTF-8 position.

This adds a printer for the high-level intermediate representation. The
regex it prints is valid, and can be used as a way to turn it into a
regex::Regex.