Repair the fuzz session after the Session trait change

[tamnd]

PR #163 changed the Session trait of rupg-func: zone now gives an Rc<dyn TimeZone>, and the new method set_setting changes a setting. The fuzz crate is a separate workspace, and the main test run does not build it. Thus the fuzz session in fuzz/src/types.rs did not compile after #163, and the nightly fuzz job would fail.

What changes:

• FuzzSession keeps its zone in an Rc and gives a clone of it from zone.
• FuzzSession::set_setting gives an internal error, as the test sessions do. No fuzz target calls set_config.

How I tested it: cargo check --all-targets and cargo fmt --check in fuzz/ pass on the stable toolchain, and cargo +nightly fuzz build -O builds wire_frontend, wire_startup, type_input and type_recv on server1. From now on, my test run before a merge also checks the fuzz workspace.