add regions, unicode idents, attributes to GtkSourceView language-spec
auto merge of #7795 : sp3d/rust/master, r=z0w0

This should be pretty self-explanatory. The most important component is region/lifetime annotation highlighting, as previously they were interpreted as character literals and would ruin the rest of the line. The attribute regex is fairly crude, but it gets the job done and there's not much within attributes that would benefit from individual highlighting, so fancier handling didn't seem worth the trouble.

The ident regex was copied from the vim highlighter.