Add picohttpparser.{c.h} from https://github.com/h2o/picohttpparser

Add picohttpparser.{c.h} from https://github.com/h2o/picohttpparser

PicoHTTPParser is a tiny, primitive, fast HTTP request/response parser.

Unlike most parsers, it is stateless and does not allocate memory by
itself. All it does is accept pointer to buffer and the output
structure, and setups the pointers in the latter to point at the
necessary portions of the buffer.