Vendor ocamlnet

[jchavarri]

Devkit only uses a small % of ocamlnet and having to maintain this package with recent versions of OCaml is adding some maintenance burden.

This PR vendors just the necessary parts of ocamlnet to keep devkit interface the same. The subset of ocamlnet is placed into an ocamlnet_lite library.

The largest parts of the vendored lib are Netconversion (by far) and Netencoding. I think there are ways to simplify more, but I took an attempt at it and I ran into some issues due to the way Enc_subset is used in encode_poly Edit: this has been simplified in 17e7d39 and e223ac1.

Another potential cleanup is to remove tstring_kind, as only String_kind is used. I already removed the memory branch, as it allowed to not bring in more modules, but left both string and bytes kinds:

devkit/ocamlnet_lite/netstring_tstring.ml

Lines 3 to 5 in 380fa87

	type _ tstring_kind =
	| String_kind : string tstring_kind
	| Bytes_kind : Bytes.t tstring_kind

TODO

• Figure out how to runt tests (libevents is missing?)
• Add some encoding / decoding examples to test suite

[rr0gi]

please add readme in ocamlnet_lite directory stating where the code comes from (url with commit hash ideal)