dims keyword for stack, unstack, unsqueeze

[CarloLucibello]

Fix #25

@darsnack @ToucheSir should we do this? I'm not 100% convinced it is necessarily better

[codecov-commenter]

Codecov Report

Merging #28 (3d632bf) into main (b45b273) will increase coverage by 0.02%.
The diff coverage is 90.00%.

@@            Coverage Diff             @@
##             main      #28      +/-   ##
==========================================
+ Coverage   89.52%   89.55%   +0.02%     
==========================================
  Files          14       14              
  Lines         401      402       +1     
==========================================
+ Hits          359      360       +1     
  Misses         42       42              

Impacted Files	Coverage Δ
src/utils.jl	96.42% <90.00%> (+0.06%)	⬆️

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[darsnack]

If I were a completely new user (i.e. no Flux experience), the new conventions seem intuitive to me (based on Base conventions). Otherwise, I don't think there's much benefit. We could always support both but this is the best place to deprecate if we want to.

[CarloLucibello]

Let's do this