isvariant(x::SumType, s::Symbol)

For an x which was created as a @sum_type, check if it's variant tag is s. e.g.

@sum_type Either{L, R} begin
    Left{L}(::L)
    Right{R}(::R)
end

let x::Either{Int, Int} = Left(1)
    isvariant(x, :Left)  # true
    isvariant(x, :Right) # false
end