Add CopyTo operator

[viceroypenguin]

This PR adds a CopyTo operator, which operates similar to Array.CopyTo or List.CopyTo. This is useful for copying data from a stream to a pre-existing array or IList<>, such as an array rented from an ArrayPool<>.

To match semantics with existing methods, this method will throw an exception if there is not enough room in the specified destination for the input data to be stored. If the consumer does not care about excess data, they can apply the Take operator before calling CopyTo to avoid the exception.

One difference between this operator and the collection methods is that this version returns an int. With the collection methods, it is easy to know pre or post how much data should be copied, because the Count/Length property is available. For IEnumerable<> this information is not as readily available, so CopyTo returns how many elements were stored in the destination.

Fixes #963

[codecov]

Codecov Report

Merging #964 (efed50c) into master (4a9041d) will decrease coverage by 0.02%.
The diff coverage is 91.37%.

@@            Coverage Diff             @@
##           master     #964      +/-   ##
==========================================
- Coverage   92.52%   92.51%   -0.02%     
==========================================
  Files         113      114       +1     
  Lines        3440     3498      +58     
  Branches     1024     1040      +16     
==========================================
+ Hits         3183     3236      +53     
- Misses        192      197       +5     
  Partials       65       65              

Impacted Files	Coverage Δ
MoreLinq/CopyTo.cs	91.37% <91.37%> (ø)

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[DanielSantaRosaZup]

💡You don't need the else statement here, because there is no more instruction after it.

[DanielSantaRosaZup]

💡 Sometimes you use { } and sometimes you don't use it in if statement. Adopt a single code style.