Improve order of arguments in H2_eio.Server.create_connection_handler for currying

[vog]

(see also anmonteiro/httpun#154)

The last two arguments of:

H2_eio.Server.create_connection_handler ... client_addr socket

should be switched to:

H2_eio.Server.create_connection_handler ... socket client_addr

This would allow for currying in Eio.Net.run_server, i.e. simplifying usage from:

let connection_handler socket client_addr =
  H2_eio.Server.create_connection_handler ... client_addr socket
in
Eio.Net.run_server ... connection_handler

to:

let connection_handler = H2_eio.Server.create_connection_handler ... in
Eio.Net.run_server ... connection_handler

@anmonteiro Would you accept a PR that fixed this?