Add support for dict[int, Iterable[str | int]] and dict[str | int, int] to stim.PauliString.__init__`

[DeDuckProject]

Added support for constructing stim.PauliString using a Dict with keys as either int (for qubit indices) or str (for Pauli).

e.g.:

stim.PauliString({0: "X", 2: "Y", 3: "X"}) == stim.PauliString("X_YX")
stim.PauliString({"X": 1, "Z: [0, 3]}) == stim.PauliString("ZX_Z")

Resolves #852

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[Strilanc]

Thanks! This is nearly mergeable.

Mainly, I think the dict[int, int] case needs to be guaranteed to be int->pauli (rather than pauli->int) to avoid ambiguity.

Secondarily, there's a few places the code is being inefficient by having a quadratic check or potential unnecessary copies.

Stylistically, the method is getting so long that this case should be extracted.

[DeDuckProject]

@Strilanc Fixed all your comments. PR is ready for merging as far as I'm concerned.
Only thing I'm debating is whether the documentation of this feature is clear enough.

[Strilanc]

Looks great! Thanks!