More general arithmetic implementations

I was recently trying to use nalgebra in conjunction with the `uom` (units of measurement) crate, which represents dimensioned quantities at the type level. A consequence of this is that you frequently get situations where things like `a * b` produce `c` where `a`, `b`, and `c` all have different types. The case that motivated me to create this issue is that I was trying to multiply a `Vector3` of one unit by a scalar of another unit, and when it didn't work, I noticed that nalgebra has a blanket impl of `Mul` only for inner types that are closed under multiplication. Forgive me if I'm wrong, but shouldn't it be possible to generalize this to arbitrary multiplication input/output types (and of course for the other arithmetic operators as well)?