Teach the PIR beta pass to handle multiple arguments at once

[michaelpj]

This has bothered me for a while. We rely on this to turn function
applications into let-bindings so that we can inline them, but because
of the way it was written it could only ever do this for at most one
argument at a time! So we could only fully-inline a function of arity N
with N rounds of the simplifier! Terrible!

Now it handles any number at once.

I wrote all the functions for this while working on
#4365, since there we
really need it, since constructors and destructors appear like
multiple function arguments rather than multiple nested
immediately-applied-lambdas. But it applies well here too, so I thought
I'd just do this improvement on the side.

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[ghost]

Nice 👍