MAINT: Implement new style promotion for `np.result_type`, etc.

This implements a new style promotion scheme for mutiple inputs
as are passed to `np.result_type`. It should be noted that
result-type heavily relies on "value-based casting" logic/promotion
logic.
This fact is inherited to many places.  When user DTypes are introduced
some of these places should potentially use "weak DType" logic instead
if the input is a Python scalar.
This is *not* yet implemented. It would be necessary to mark these
the same way as is currently done for `np.result_type`.

This type of "marking" will generally be necessary in a few places,
even if may get away with just switching the logic everywhere.

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