`SIMD{n}: ElementaryFunctions` conformances

[dan-zheng]

ElementaryFunctions was added to apple/swift with conformances for SIMD types. It seems those conformances were dropped when ElementaryFunctions was moved to apple/swift-numerics.

Is it reasonable to re-add SIMD{n}: ElementaryFunctions conformances to apple/swift-numerics?

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Context: tensorflow/swift#404

[stephentyrone]

Yup, this is a very reasonable thing to add, I'll take a look shortly.