sparse constructor, zero value to kwarg, constructors with splatted dims

[mtfishman]

Fixes #40, fixes #42.

Also turns the function used by getunstoredindex into a keyword argument called getunstored (before it was an argument, which wasn't good for constructors where the dims are splatted).

To-do

• More tests.

[codecov]

Codecov Report

Attention: Patch coverage is 68.49315% with 46 lines in your changes missing coverage. Please review.

Project coverage is 74.88%. Comparing base (4e94eea) to head (d01eb1f).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/oneelementarray.jl	63.20%	39 Missing ⚠️
src/sparsearraydok.jl	71.42%	4 Missing ⚠️
src/abstractsparsearray.jl	88.46%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #43      +/-   ##
==========================================
+ Coverage   71.49%   74.88%   +3.39%     
==========================================
  Files           7        7              
  Lines         435      450      +15     
==========================================
+ Hits          311      337      +26     
+ Misses        124      113      -11     

Flag	Coverage Δ
docs	35.71% <6.84%> (-1.56%)	⬇️

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