Avoid using the cohttp transfer_io module

[djs55]

Unfortunately the cohttp module uses strings throughout which means we must

• receive a Cstruct.t
• copy into a string
• copy into another Cstruct.t

while proxying HTTP bodies. This patch inlines the proxying code to remove these 2 unnecessary copies.

This patch also adds support for trailing headers and chunk extensions as a side-effect.

Signed-off-by: David Scott [email protected]