This has now been approved as a language feature and no longer needs
a `rustc_` prefix.
Also change the `contracts` feature to be marked as incomplete and
`contracts_internals` as internal.
Instead of parsing the different components of a function signature,
eagerly look for either the `where` keyword or the function body.
- Also address feedback to use `From` instead of `TryFrom` in cranelift
contract and ubcheck codegen.
The extended syntax for function signature that includes contract clauses
should never be user exposed versus the interface we want to ship
externally eventually.
