Warn for conditional function definition in local scopes

[pfitzseb]

function f()
    if rand() < 0.5
        g(x) = x
    else
        g(x) = 2x
    end
    g
end

should be written as

function f()
    if rand() < 0.5
        g = x -> x
    else
        g = x -> 2x
    end
    g
end

instead.