The intel, arm32, and arm32 targets were only defined in the meta
language previously. Add Rust implementations too.
This is mostly boilerplate, except for the unit tests in the
registers.rs files.
The 'lib/cretonne' directory will be the new root of a stand-alone
cretonne crate containg both Python and Rust sources.
This is in preparation for publishing crates on crates.io.