words: extend usimdN to allow returning a word of different size

[aqjune]

This is a simple pull request that extends the usimdN functions in Library/words.ml to allow returning a word whose size is different from the input word size.

The updated functions are to be used in an upcoming new pull request in awslabs/s2n-bignum that adds formal definitions of new SIMD operations.

[jrh13]

Thanks, a useful generalization, I'll merge this in.