Add SpillHead

[atifaziz]

Implementation for SpillSpanSpillHead proposed in #752.

[codecov]

Codecov Report

❌ Patch coverage is 94.48819% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.70%. Comparing base (e52d278) to head (25c256a).
⚠️ Report is 27 commits behind head on master.

Files with missing lines	Patch %	Lines
MoreLinq/SpillHead.cs	94.48%	5 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #753      +/-   ##
==========================================
- Coverage   93.22%   92.70%   -0.53%     
==========================================
  Files         112      113       +1     
  Lines        3395     3522     +127     
  Branches      960     1025      +65     
==========================================
+ Hits         3165     3265     +100     
- Misses        214      239      +25     
- Partials       16       18       +2     

Flag	Coverage Δ
	92.70% <94.48%> (-0.53%)	⬇️

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