Improve merging of Char parsers

[johnynek]

1. If we have a single CharIn parser, we don't need to compute the union, just reuse it (which also saves recomputing the range of characters)
2. return a Set in the union function, not a List, which can return a large list if you have duplicates in your set.

[codecov-commenter]

Codecov Report

Merging #291 (26f2b27) into main (0039bb2) will increase coverage by 0.45%.
The diff coverage is 100.00%.

❗ Current head 26f2b27 differs from pull request most recent head f1a3422. Consider uploading reports for the commit f1a3422 to get more accurate results

@@            Coverage Diff             @@
##             main     #291      +/-   ##
==========================================
+ Coverage   95.98%   96.43%   +0.45%     
==========================================
  Files           8        8              
  Lines         997     1011      +14     
  Branches       88       81       -7     
==========================================
+ Hits          957      975      +18     
+ Misses         40       36       -4     

Impacted Files	Coverage Δ
core/jvm/src/main/scala/cats/parse/BitSet.scala	85.71% <100.00%> (ø)
...shared/src/main/scala/cats/parse/LocationMap.scala	97.05% <100.00%> (+1.22%)	⬆️
core/shared/src/main/scala/cats/parse/Parser.scala	96.32% <100.00%> (+0.49%)	⬆️

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[regadas]

could we have a version of union with Iterator and avoid the copy in minBs?

[johnynek]

we certainly could. I wasn't sure it was worth it because we already iterate through the number of parsers regularly, but the characters inside the parsers (which we have to iterate when allocating a new CharIn to compute the ranges) could be more expensive, which was what I was targeting here.

Do you want me to address this before merging?

[regadas]

We can address this as a follow-up for sure!