Dispatch on Pair

[jw3126]

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|__/                   |  x86_64-linux-gnu

julia> f{K}(::Pair{K}) = 1
f (generic function with 1 method)

julia> f{K, V}(::Pair{K, V}) = 2
f (generic function with 2 methods)

julia> f("a"=>123)
1

[JeffBezanson]

I can already confirm this will be fixed by #8974, after which the two signatures are type-equal and the second definition overwrites the first (with usual warning).

[JeffBezanson]

Dup of #18892