Tweak `FlatPat::new` to avoid a temporarily-invalid state
It was somewhat confusing that the old constructor would create a `FlatPat` in a (possibly) non-simplified state, and then simplify its contents in-place.
So instead we now create its fields as local variables, perform simplification, and then create the struct afterwards.
This doesn't affect correctness, but is less confusing.
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I've also included some semi-related comments that I made while trying to navigate this code.
7e1489cf63