Add jsmn JSON parser to bmk-core.

One could debate whether we need it in bmk-core or rumprun-base, but
given that it's extremely small and the only external dependency is to
NULL (apart from size_t which I converted to unsigned long), we might
just as well put it into core and see if in-core uses pop up.

MIT licensed, from http://zserge.com/jsmn.html