[PROPOSAL] Define fixed-point types in `cudf::` namespace

[bdice]

Fixed-point (decimal) types are defined in a top-level namespace numeric and not in the namespace cudf.

This shows up in the stringified type name, and violates the expectation that packages should probably keep all their definitions in a namespace owned by the package, i.e. cudf::.

See additional discussion: #12158 (comment)

I propose moving the fixed-point code to a namespace cudf::fixed_point and refactoring as needed.

[harrism]

Seconded.