client support

[hannesm]

this PR includes:

• accept a version string without \r\n but only with \n (this is what my FreeBSD OpenSSH server sends)
• build system changes (use dune for now)
• avoid deprecated Cstruct.set_len function
• an initial client implementation (with lots of limitations, but sufficient for running git+ssh) - i.e. only rsa key authentication, no rekeying, ...
• an awa_mirage sublibrary which exposes the client implementation to be used by [WIP] ssh client support in conduit_mirage ocaml-conduit#296 (and initial ssh (client) support using awa-ssh ocaml-git#362)

[hannesm]

since this extends awa (which is unreleased), there's no harm merging it into master.