Rust: SSA additions

[paldepind]

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This PR makes two small improvements to the SSA:

• Mutable variables that are captured are now supported. Immutable variables where already supported. I don't think allowing mutable variables as well pose much of a problem, as they don't introduce the same issues that mutable borrows do.

  The only change I made to support this was to insert pseudo reads upon exit from a closure where captured variables are written to, which I observed was done in the Ruby SSA implementation. There might be other necessary changes that I've missed, so please double check 🙏

• Mutable variables are now only excluded from SSA when they are mutably borrowed (&mut) whereas before merely immutably borrowing them (&) excluded them. I don't think immutable borrows pose any problems for the SSA construction.