[Rust] Allow mapping between SSA and non-SSA forms of LLIL instructions / expressions

[bdash]

For LowLeveLIL{Expression,Instruction}<M, NonSSA> this adds:

pub fn ssa_form<'a>(&self, ssa: &'func LowLevelILFunction<M, SSA>) -> …

For LowLeveLIL{Expression,Instruction}<M, SSA> this adds:

pub fn non_ssa_form(&self, non_ssa: &'func LowLevelILFunction<M, NonSSA>) -> …

One thing I wasn't sure about is having these validate that self.function and the passed-in function represent the same underlying function object. Is there an easy way to verify that?

[bdash]

The latest push fixed the clippy warnings the earlier iteration had introduced.

[emesare]

Cherry picked on dev with 82a211d. Thank you!