API,MAINT: Rewrite promotion using common DType and common instance

This defines `common_dtype` and `common_instance` (only for parametric
DTypes), and uses them to implement the `PyArray_CommonDType` operation.

`PyArray_CommonDType()` together with the `common_instance` method
then define the existing PromoteTypes.

This does not (yet) affect "value based promotion" as defined by
`PyArray_ResultType()`.  We also require the step of casting
to the common DType to define this type of example:
```
np.promote_types("S1", "i8") == np.dtype('S21')
```
This steps requires finding the string length corresponding to
the integer (21 characters).  This is here handled by the
`PyArray_CastDescrToDType` function.  However, that function
still relies on `PyArray_AdaptFlexibleDType` and thus does not
generalize to arbitrary DTypes.

See NEP 42 (currently "Common DType Operations" section):
https://numpy.org/neps/nep-0042-new-dtypes.html#common-dtype-operations

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