scale and scale! have redundant methods

[jiahao]

The base library contains many different methods for scale and scale! which look like they are pre- and post-multiplicative variants, but are actually not. For example:

scale(x::Number, y::Vector) = x * y
scale(x::Vector, y::Number) = x * y

These methods are redundant when the product is commutative, and they are wrong when it is not (e.g. quaternions, which are clearly <: Number).

We should either

1. Delete half of these methods so that scale-ing only means either pre- or post-multiplication, or
2. Fix the existing methods to not assume commutativity, i.e. distinguish between pre- or post-multiplications.

[StefanKarpinski]

Fixing it to not assume commutativity seems best.

[sarvghotra]

Made a pull request #14402.

[pkofod]

Fixed by #14425 ?

[ivarne]

Seems like it was.