stricter set type match, implicit conversion for literals

[metagn]

fixes #18396, fixes #20142

Set types with base types matching less than a generic match (so subrange matches, conversion matches, int conversion matches) are now considered mismatching, as their representation is different on the backends (except VM and JS), causing codegen issues. An exception is granted for set literal types, which now implicitly convert each element to the matched base type, so things like s == {'a', 'b'} are still possible where s is set[range['a'..'z']]. Also every conversion match in this case is unified under the normal "conversion" match, so a literal doesn't match one set type better than the other, unless it's equal.

However {'a', 'b'} == s or {'a', 'b'} - s etc is now not possible. when it used to work in the VM. So this is somewhat breaking, and needs a changelog entry.

[github-actions]

Thanks for your hard work on this PR!
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