Fix bugs in Batch

[viceroypenguin]

This PR fixes three bugs in the Batch operator and adds tests to verify the fixes.

Bugs:

• The size parameter of the buffered implementation should be allowed to be equal to array.Length.
• BatchListIterator should return the right Count when supplied an empty sequence.
• The buffered implementation returned the wrong value for an empty source sequence.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (13d70fc) 93.02% compared to head (9e1ffda) 93.03%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #587      +/-   ##
==========================================
+ Coverage   93.02%   93.03%   +0.01%     
==========================================
  Files         255      255              
  Lines        8140     8140              
  Branches     1618     1620       +2     
==========================================
+ Hits         7572     7573       +1     
+ Misses        359      358       -1     
  Partials      209      209              

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