Implement Tail-Call Elimination

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CL doesn't guarantee TCE (and some implementations famously don't have it), so I think we should try to implement at least a few idiomatic cases of it, like self-calls.

Cons:

• A partial TCE implementation may not really render benefits in practice.

• Most used Lisps of today have TCE in the right settings.

• I think this would complicate our IR and codegen.