scalar_prod implementation

[sebasv]

I am missing a scalar_prod function. on ArrayBase. This seems straightforward to implement (almost blindly copy/paste scalar_sum), so I will be submitting a pull request. I am not familiar enough with computer engineering to say if an unrolled_prod is a valuable addition as well, so I will just include it in the PR awaiting feedback from someone with more background!

[jturner314]

I am not familiar enough with computer engineering to say if an unrolled_prod is a valuable addition as well

If you're interested, this is a pretty good article for background, even though it's talking about C++ with the MS compiler instead of Rust with LLVM. Basically, in order to allow the compiler to perform the sum/product using SIMD even with strict floating-point behavior, it's helpful to explicitly unroll and reorder the sum/product manually.

[sebasv]

That seems like a very nice read, thank you!

[jturner314]

Implemented in #505.