MezzoWeb.

- Hide advanced options by default.
- Fix the interpreter.

For some reason, print_endline is improperly compiled, meaning we fail to see
the output when calling the Mezzo [print] function.
MezzoWeb.

Properly handle output:
- write the special //Provides: clauses for js_of_ocaml, so that output is
properly redirected onto the HTML console
- a few cleanups here and there too.
MezzoWeb.

Don't memoize the .mz and .mzi for the special "::toplevel" file (since it
changes...).