Toplevel: add a simple cohttp based server

Provides just enough to enable topfind in the js_of_ocaml toplevel.

You may find this useful for testing dynamic loading with js_of_ocaml.

Note; xmlHttpRequests are generated regardless of the presence of a server that can support them which might not be the behaviour you want.  You might also want support of command line options and serving multiple directories as this feature and also enables `open_in` to read arbitrary files from the filesystem.