Type signature for `Pair` methods are out of date

[ab5tract]

Problem or new feature

For Pair, the kv, pairs, antipairs, keys, and valuesmethods are all indicated to return aListbut in reality they returnSeq`.

Also, unnecessary parentheses are included in several of the examples, eg my $p = (Raku => "d");

See: https://docs.raku.org/type/Pair

Suggestions

Adjust the documentation to reflect the Seq return values.

[coke]

Note: we should update xt/check-signatures.rakutest so it can catch failures of this type.

[coke]

Raku/problem-solving#499