Move SparseArrays dependency to extension

[j-fu]

This should help to resolve SciML/LinearSolve.jl#573

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 95.66%. Comparing base (7810cf5) to head (c8c9a23).

Files with missing lines	Patch %	Lines
src/lazyoperations.jl	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #365      +/-   ##
==========================================
- Coverage   95.72%   95.66%   -0.06%     
==========================================
  Files          17       18       +1     
  Lines        3204     3210       +6     
==========================================
+ Hits         3067     3071       +4     
- Misses        137      139       +2     

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[dlfivefifty]

"my_" is a very bad naming convention. I have no idea what these are for!

[j-fu]

Well the idea is to have some issparse and nnz methods which defaults to false in the case SparseArrays is not loaded, and is extended with the ones from SparseArrays in the extensions. Do you agree with the general approach ?

Instead of "my_" we could use "", "tentative" "local_" "provisional_" "preliminary_" ...