Optimise FallbackIfEmpty

[julienasp]

This PR improves the FallbackIfEmpty performance in the case of collections.

Fixes #367

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02 🎉

Comparison is base (56f77c0) 91.44% compared to head (1879e0c) 91.47%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #464      +/-   ##
==========================================
+ Coverage   91.44%   91.47%   +0.02%     
==========================================
  Files         245      245              
  Lines        7936     7949      +13     
  Branches     1608     1611       +3     
==========================================
+ Hits         7257     7271      +14     
+ Misses        461      460       -1     
  Partials      218      218              

Impacted Files	Coverage Δ
Source/SuperLinq/FallbackIfEmpty.cs	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[viceroypenguin]

Check out this test for an example of why we cannot evaluate collection count when the operator is called.

In further consideration, I wonder if I was forgetting that when I write the issue. In theory, this could be abstracted to an If() that selects the right IEnumerable, but in practice, don't know that it's an actual optimization.

I'm going to leave this open for a few days while I ponder whether to ask you to improve this or to ask me to stop coming up with bad ideas... :)

[viceroypenguin]

@julienasp I think I would like to have FallbackIfEmpty implemented as a CollectionIterator. You can review DoIterator for a minimum implementation and Insert method for example of how it can be checked for two IEnumerable<>. I think the GetEnumerable() and CopyTo() methods can be implemented.

[julienasp]

@julienasp I think I would like to have FallbackIfEmpty implemented as a CollectionIterator. You can review DoIterator for a minimum implementation and Insert method for example of how it can be checked for two IEnumerable<>. I think the GetEnumerable() and CopyTo() methods can be implemented.

perfect i'll work on that this weekend

[viceroypenguin]

Thanks! Looking good so far.

[viceroypenguin]

I think this is the last round of changes. 🤞🏼

[viceroypenguin]

@julienasp Thanks!