Header refactoring, reworking, etc.

--HG--
rename : src/libhttp/headers.rs => src/libhttp/headers/mod.rs
Add missing files of last commit.
Add more missing files of last commit.
Add host field back.
General fix-up, etc. Still doesn't compile: ICE.

There were lots of problems with what was committed last; some were due
to upstream changes (std::ascii::to_ascii_lower changed from a function
to a trait method), others due to writing lots of code without a rust
compiler on hand for sanity checking. These are now pretty much all
fixed up. But alas, I've made me some ICE again: "ty_self not found in
substs". I suspect it's to do with my fancy static trait methods on
generic types (HeaderEnum). Now I get to hunt that down and report it.