axp(b)y! and mul! (scalar multiplication) with mixed argument types

[Jutho]

It would be good (and I would hope not too difficult) to support muting LinearAlgebra methods that are related to taking linear combinations of arrays:
mul!(y, x, scalar)
axpy!(scalar, x, y)
axpby!(scalar, x, scalar, y)
where scalar and also x and y can have different types.

Currently these operators fall back to their generic implementation in Base, which of course uses scalar indexing and is thus forbidden.

Not sure if this needs to go into CuArrays.jl or in GPUArrays.jl.

I encounter this in KrylovKit.jl, where complex valued linear combinations of the real-valued Schur vectors are taken in order to build the eigenvectors: Jutho/KrylovKit.jl#15

[maleadt]

I think many of these are implemented now, with scalar input types having been widened to Number. If there's any missing, feel free to open another issue.

[Jutho]

Great; thanks!