Avoid constructing MulAddMuls on Julia v1.12+

[dkarrasch]

This is a companion PR to JuliaLang/julia#52439. Basically, it avoids the construction of MulAddMuls and simply passes the factors alpha and beta forward.

[dkarrasch]

I changed things such that there are two methods of each signature type: first a legacy method, that has MulAddMuls in their final argument. Those get destructured and the call is forward to the second method, that has alpha, beta in their final places. On older versions, the pathway is via the first one, on upcoming versions after the above mentioned PR is merged, the second method will simply overload the upstream generic_matmatmul! and step in the place of the first method directly in the call chain. Altogether, this should be non-breaking, so if tests pass, this could be merged and released, and then we see how it goes on current Julia master.

[maleadt]

Thanks!