Update toolchain

[nilehmann]

• The way rustc_driver dynamic library is distributed changed here. Had to move config into a new crate that doesn't depend on rustc such that binaries in flux-bin also don't dependent on them.
• After the update lalrpop is giving a future incompatibility warning. There's a PR fixing it, but the crate is not getting much maintenance these days. Maybe we should start thinking about moving to a different parser library.