proof of concept for mul_tr

[fangs124]

I saw issue #1254. Attempted to implement it following the implementation for tr_mul. I'm not quire sure why the original hidden function was called xx_mul_to_unit (specifically the xx part), but I made a corresponding function yy_mul_to_uninit.

Opening this pull request mainly to gather feedback, and if all is good I should be able to write other variations of the functions in ops.rs such as mul_ad, mul_tr_to, etc.

I also also wanted to add a test similar to tr_mul_is_transpose_then_mul was for tr_mul, except for the new mul_tr function. I'm not used to writing tests, and especially not the proptest library. I was able to write a test, but I had to resort to physically transposing a column vector for the test.

Not really a programmer by trade, new to github. Feedback welcomed.