Disambiguate mul! for matvec and matmat through indirection

[jishnub]

This forwards mul! for matrix-matrix and matrix-vector multiplications to the internal _mul!, which is then specialized for the various array types. With this, packages would not encounter any method ambiguity when they define

mul!(::AbstractMatrix, ::MyMatrix, ::AbstractMatrix, alpha::Number, beta::Number)

This should reduce the number of methods that packages need to define to work around ambiguities with LinearAlgebra.

There was already an existing internal function named _mul!, but the new methods don't clash with the existing ones, and since both sets of methods usually forward structured multiplications to specialized functions, it's fitting for them to have the same name.

[dkarrasch]

IIUC, then this moves the "internal LinearAlgebra" disambiguation to _mul!. Is that correct? So packages don't need to disambiguate with the matrix subgroups BandedMatrix and *Triangular? I think this is worth a NEWS entry, to explain which type of ambiguity used to exist, and is now removed. I suspect one of the main beneficiaries will be ArrayLayouts.jl.

[jishnub]

Yes, you're right. I suspect FillArrays and other similar array packages that support linear algebra would also benefit from this, but none more than ArrayLayouts.