remove gmp from dependency cone

[hannesm]

since asn1-combinators 0.3.x doesn't need zarith anymore, let's get rid of the gmp dependency from builder. the only challenge is the builder-server using http-lwt-client (which depends on tls). we may strive for two packages, builder-worker and builder-full (client / server)? or push the http stuff to calling an external program (hurl.exe/curl)?

what do you think?

[reynir]

I think it is worth investigating. Metrics that could be interesting to compare is: build times and binary sizes.