Five arg mul! for UniformScaling and improvement in exp!

[jarlebring]

This PR implements a five argument mul! for UniformScaling as third argument and second is Number (and conversely second Number and third UniformScaling).

In particular, it provides the possibility to do in-place add of UniformScaling-objects. See the discussion in https://discourse.julialang.org/t/uniform-scaling-inplace-addition-with-matrix/

The in-place version of A += s*I is

mul!(A,true,s*I,true,true)

As an example / use-case, I have adapted exp! (after #40668) where it was needed four times. Compare:

julia> A=randn(100,100); A=0.001*A/opnorm(A,1);

julia> @btime exp_org!(copy($A));
  740.067 μs (21 allocations: 705.69 KiB)

julia> @btime exp_new!(copy($A));
  702.532 μs (17 allocations: 549.28 KiB)

julia> A=randn(100,100); A=2.3*A/opnorm(A,1);

julia> @btime exp_org!(copy($A));
  1.138 ms (33 allocations: 942.28 KiB)

julia> @btime exp_new!(copy($A));
  1.053 ms (27 allocations: 940.38 KiB)

[dkarrasch]

I think we should additionally test the beta = 0 and the alpha = 0 case, and then this is ready to go.

[jarlebring]

When we are still at it, I will add mul!(::AbstractMatrix,::UniformScaling,::Number,::Number,::Number).

[dkarrasch]

LGTM.