Temporarily exclude crates that require nightly

This is a prelude to immunant#1220 . In
the future, these crates can be split into their own workspace, in order
to build them with nightly. In the meantime, exclude them, move the
rust-toolchain.toml files into them, remove it from the top level, and
start building the rest of the c2rust workspace with stable Rust
instead. This will unblock further changes, such as migrating c2rust to
current dependencies.

Author: joshtriplett