Add matrix-vector product

[ocramz]

Add matrix-vector product (#>) in Numeric

• [ ] Bump up the version in cabal file
• [x] Any changes that could be relevant to users have been recorded in the CHANGELOG.md
• [x] The documentation has been updated, if necessary.
• [ ] Property tests or at least some unit test cases been added for all new functionality.
• [ ] Link to any issues that might be related to this Pull Request.

[lehins]

Thank you again for this PR. The actual function implementation is perfect, there are a couple of minor changes I'd like to suggest:

• It is better to return D array type, instead of any Construct, it simplifies type inference for the user, without hurting the performance or usability. It is always possible to call computeAs on the result to get a different representation.
• I haven't released massiv-0.5.2 yet so the version bump wasn't really necessary. I just added the functions you were asking about couple of days ago lowerTriangular and upperTriangular, but have not got a chance to do cut the release it yet.