handle uniform scalings in compositions of maps explicitly

[dkarrasch]

This is probably an incredibly dirty hack, but it does all the things we were considering in the context of #57, namely:

• it's type stable, since it doesn't touch the constructors at all, but rather handles everything within the mul! as suggested by @Jutho in make A*I return A #63;
• it finishes earlier when one of the maps in a composition is a scaling by zero;
• it skips uniform scalings by one, no matter in which position they occur, and also no matter of what exact type the one is, i.e., this is not restricted to Bools.

[coveralls]

Coverage decreased (-0.7%) to 97.716% when pulling acfb701 on identity into c435822 on master.

[codecov]

Codecov Report

Merging #69 into master will decrease coverage by 1.7%.
The diff coverage is 72.41%.

@@            Coverage Diff             @@
##           master      #69      +/-   ##
==========================================
- Coverage   98.62%   96.91%   -1.71%     
==========================================
  Files           8        8              
  Lines         435      454      +19     
==========================================
+ Hits          429      440      +11     
- Misses          6       14       +8

Impacted Files	Coverage Δ
src/composition.jl	88.88% <72.41%> (-9.5%)	⬇️

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[Jutho]

The reason for assigning dest this way is to make sure that the eltype of dest is fixed (to that of y) such that it can hopefully be reused later on if there are more than 2 linear maps. This might fail with the new lines of code below.

[Jutho]

Thanks for working on this.

While the tests turn green, I think I could see cases where this fails. Also, there is still copying going on with identity linear maps, so I think a more involved change is required if we really don't want to do any extra work for uniformscalingmap with lambda = 1.