Replace Chunks with simple ByteVectors.

This demotes little-used trailers to a second-class, unsavory API.  In
exchange, we no longer need to wrap ByteVector and avoid a lot of
pattern matching.
Store trailer headers as an attribute.
small fixes
Merge remote-tracking branch 'origin/topic/bryce-byte-vector' into topic/byte-vectors
Merge branch 'topic/byte-vectors' into develop