Fix websocket conflict, update deprecated Fmt functions

[zshipko]

Fixes #203 by renaming websocket.ml and websocket.mli to graphql_websocket.ml and graphql_websocket.mli

[zshipko]

I don't think the test failure is related to these changes. But I tried adding core_kernel to the graphql-async opam file but that didn't seem to fix the issue.

Edit: Looks like Core_kernel is now deprecated so I just updated the test to use Core instead

[andreas]

Thanks!