Parsing / ignoring headers

[craff]

Currently, only official headers are taking into account and only the value of those headers (no its name) is allocated as a string.
There are two possible amelioration:

• associate a parsing function to each header, which would for instance avoid allocation all allocation for Content-Length.
• ignore the header that are not used by a Route. We could imagine something using GADT and mostly automatic for the programmer that would give the list of headers used by a route, all others headers could be completely ignored.

Remark: header parsing and the resulting allocation has been a very big source of speed improvement and much more is possible.