Preserves the fn accessors within the Monero crates so that we can use statics
in some cfgs yet not all (in order to provide support for more low-memory
devices) with the exception of `H` (which truly should be cached).
Adds a Rust toolchain file to be less disruptive to developers who don't keep
their toolchain synchronized (by now having rustup automatically synchronize).
Hopefully helps resolve how +nightly clippy may pass for the coordinator, yet
building would fail due to stable's (hopefully prior?) failure to model some
async functions re: Send/Sync.
Also adds rust-src as a component in preparation of
https://github.com/paritytech/polkadot-sdk/pull/2217
