have dropdims default to dropping all singleton dimensions

[lstagner]

I honestly just got really tired of writing the same patterndropdims(sum(x,dims=(1,2)),dims=(1,2)). This makes it so that dropdims will just drop the dims.

[timholy]

See #22000 and https://stackoverflow.com/questions/52505760/dropping-singleton-dimensions-in-julia/52515948?noredirect=1#comment91975665_52515948.

But of course you can define dropdims1 in your own code if you like.

[lstagner]

Ok so the basic problem is that the generated keyword argument isn't type stable because it can't know the length of the tuple a priori.