Much like we optimize to_ascii_lowercase.
This also fixes a bug in Servo where attr() rules with an unknown namespace
prefix are parsed, which is wrong.
This more concrete wrapper type can write a prefix the very first time something
is written to it. This allows removing plenty of useless monomorphisations caused
by the former W/SequenceWriter<W> pair of types.
… rather than the start location of the current construct.
This likely places the error just *after* of the unexpected token
whereas before would be best, but that’s likely a much bigger change.
See https://bugzilla.mozilla.org/show_bug.cgi?id=1378861
The @font-feature-values declaration identifiers are asci case insensitive.
We should convert all of them to lowercase to be able to reduce the same
declarations. Also gecko stores them as lowercase in gfxFontFeatureValueSet.
