[Rust] Support retrieving use / def of SSA registers in LLIL

[bdash]

This adds:

impl<M: FunctionMutability> Ref<LowLevelILFunction<M, SSA>> {
    pub fn get_ssa_register_uses<R: ArchReg>(
        &self,
        reg: LowLevelILSSARegisterKind<R>,
    ) -> Vec<LowLevelILInstruction<M, SSA>> { … }

   pub fn get_ssa_register_definition<R: ArchReg>(
        &self,
        reg: &LowLevelILSSARegisterKind<R>,
    ) -> Option<LowLevelILInstruction<M, SSA>> { … }
}

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I'm not clear whether this handling of partial registers is correct. There's not a lot of information on their semantics and I couldn't immediately find how Python or C++ handle them.

[emesare]

Cherry picked on dev with abc355d. Thank you!