Move SparkMurmurHash3_32 functor.

[bdice]

Description

This PR moves the SparkMurmurHash3_32 functor from hash_functions.cuh to spark_murmur_hash.cu, the only place where it is used. This is a pure move, with one small exception to avoid compiler warnings about unused members of the hash functor template instantiations for nested types. I refactored the class template to disallow nested types for the hash functor and removed those specializations using CUDF_UNREACHABLE, rather than allowing type dispatching to create template instantiations that have no defined use. (Nested types are being handled by the custom device row hasher in spark_murmur_hash.cu, and require some state information that cannot be easily carried in the functor itself.) I am planning to do further refactoring later, but wanted to separate this "pure move" as much as possible.

Part of #10081.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[bdice]

rerun tests

[codecov]

Codecov Report

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[rwlee]

+1

[bdice]

@gpucibot merge