[fea-rs] ensure first wins when specific kern rules conflict

[anthrotype]

Thanks @simoncozens for reporting this and proposing a fix, I just applied that and added a test for confirm.

https://adobe-type-tools.github.io/afdko/OpenTypeFeatureFileSpecification.html#6bii-enumerating-pairs

Specific pairs generated by an enum rule may be overridden by specifying preceding single pairs. Because of this case, it is not an error when specific kern pairs conflict because they have the same glyphs. When specific kern pair rules conflict, the first rule specified is used, and later conflicting rule are skipped.

Fixes #550