Integrate fuzzers with workspace, somehow?

Currently, each fuzzing target gets its own Cargo.toml, and its own Cargo.lock, and its own target directory; it isn't part of the main workspace.

It might be cool to fix this, to save on our resources when we're building fuzzers. But some wrinkles arise:

• Notably, cargo-fuzz requires +nightly.
• Probably, we don't want to build all the fuzzers by default when somebody says "cargo build" from the top of the workspace.