Complete binary operators for dense matrix/vector types

This was discussed a bit on the Developer Meeting 2018. The topic is still controversial, so will need a bit of benchmarking.

TODO:

• Figure out which operators exactly are missing.
• Figure out how people would be using those operators.
  (Several users have actually implemented these operators in their own libs, those could be used to get an idea.)
• Benchmark. Figure out which kind of uses do give different performance.
  (I started with that a little here joe/dense-types-benchmark based on google benchmark lib, though there isn't much dense types specific yet)
• Decide whether to go ahead with implementation.
• Implement.