fix possible NULL deref on gophermap' serialize_link it's just a typo: instead of looking for the end of the string pointed by path, if look for the end of uri. reached that point, path is NULL. reported by a gcc 11 warning: "argument 1 null where non-null expected"

replace has_prefix with strncmp

partial revert of "abstract over evbuffers" partial revert of commit d54dd8160b88709f3d243e1410a781e5de7fc187. We don't really need an abstraction over "printf-like" things. I can just use plain old FILEs. open_memstream (which conforms to POSIX.1) is what I should have used from the beginning in my mcache implementation.

parser/serialize: abstract over evbuffers the serialize "method" will soon be used in other context where using evbuffers would be awkward. Introduce a small abstraction over it: a printfn (print function) fprintf-like.

move parsers to their own subdir