Optimizations when alpha or beta is 0

[maleadt]

mul! is commonly invoked with beta = 0, which means we shouldn't load from C unconditionally.

[thomasfaingnaert]

To handle that case, we probably can just add a ZeroLayout{T} that always returns a 0, and pass that as C layout to the Config if beta == 0.

[maleadt]

Works beautifully. I'll make that part of a BLAS rework PR I'll create some time this week.