Most implementations of md5 internally consist of 3 functions: md5_init -
which initializes internal context, md5_update - which accepts portions of
data and processes them and md5_final - which finalizes the hash and
releases the context. These roughly suit aggregate's internal functions
(SFUNC and FINALFUNC, md5_init is probably to be called on first actual
input). Since performance is important for you the functions should be
written in low-level language as C, to me it doesn't look difficult to take
some C md5 module and adapt it to be an aggregate... though it's not like I
would do this easily myself :)