It is used in the case where a variable-length slice pattern is used to
match on an array of known size. This allows considering only those
entries in the array that are captured by one of the patterns.
As a side-effect, diagnostics improve a bit for those cases.
This improves error messages by indicating when slices above a certain
lengths have not been matched. Previously, we would only report examples
of such lengths, but of course never all of them.
I took most tests that were testing only for match exhaustiveness,
pattern refutability or match arm reachability, and put them in
the same test folder.
