Add http::chunked_encoder:

This transforms a ConstBufferSequence into a new ConstBufferSequence whose
data is encoded according to the Content transfer encoding rules of RFC2616.
The implementation does not copy any memory but also does not coalesce
buffers into larger ones if the input data is in small pieces.
Reviewers: @rec @donovanhide
:-(

Suddenly unilaterally decided you're using a new coding style is frustrating, particularly since each code review from you has a variety of styles.  Can't you just pick one and stick with it?  Perhaps one that's close to the rest of the codebase?