Add hash for SparsePoly

[lgoettgens]

Resolves one issue described in #1689.

Note that this still not fulfils the contract that == objects have the same hash due to the ad-hoc comparisons to the base ring and to Int etc. But since this is the same case for "normal" polys, I would ignore this.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 86.90%. Comparing base (9c828f7) to head (a7ee78f).
Report is 1 commits behind head on master.

Files	Patch %	Lines
src/generic/SparsePoly.jl	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1691      +/-   ##
==========================================
- Coverage   87.34%   86.90%   -0.45%     
==========================================
  Files         116      116              
  Lines       29713    29693      -20     
==========================================
- Hits        25954    25805     -149     
- Misses       3759     3888     +129     

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