[Rust] Support SSA form of LLIL

[bdash]

What is the feature you'd like to have?
Support for the working with the SSA form of LLIL via the Rust API.

rust/src/low_level_il has some references to the various SSA instructions and expression types which made me think it worked, but it doesn't handle them correctly. That's not as big a problem as it seems though as there doesn't appear to be any way to get the SSA form of a function.

There's a lot of analysis that's challenging to implement without the SSA form.

[emesare]

Working on it on the cleanup branch: fe7b9b5