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.

switch the cache to the new parser_serialize API

crank up cache limits delete pages that are older than one hour, and scan the cache only once every five minutes. The previous defaults were a bit too much conservative.

drop useless struct instead of keeping the struct mcache that's private to mcache.c and used only once, just define its fields as (static) variables. While here, mark as static also gemtext_prefixes.

timer-based cache removal: delete cached pages older than 15 minutes

cache: keep track of the total size and number of cached pages

mcache_tab: free previous copies of that page

move util.c declarations in utils.h it's not spelled `util.h' because otherwise it'd get included instead of /usr/include/util.h

de-duplicate hash_* function: collect them in util.c

save parser name across mcache_tab/mcache_lookup

simplify the caching API - don't expose the evbuffer where we store the cached page - cache to/from tabs instead of buffers

add an in-memory cache store see github issue #1