[Mercury Feature request] Allow Enum math

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Enum math:

    <Flags>
    Public Enum a As Short
        empty = 0
        a = 1 '001
        b = 2 '010
        c = 4 '100
    End Enum

        Dim e As a = a.empty
        e += a.a 'sets the a flag if it's not on yet
        e += a.b 'sets the b flag if it's not on yet
        e -=  a.a 'unsets the a flag if it's on

so, in short operators on flag enums