Remove collections trait implementations.

Rust has removed them due to collections reform RFC:

RFC: https://github.com/rust-lang/rfcs/pull/235/files
Commit:
https://github.com/rust-lang/rust/commit/21ac985af44f4e2470ef6f4c0eb4d72daf5a6497
Re-add len() and clear() as inherent Headers methods.
Add len() test with redundant header.
Merge pull request #95 from jamwt/remove_coll_traits

Move len/clear from trait implementations to inherent methods