redefining constants

[skariel]

It would be good if it could catch the following:

lintstr("""
const a=1
const d=11
f=12
function g()
    const a=2
    const b = 1
    function f()
        a=4
        b=5
        const c=1
        c=2
        global d
        d=12
        global f
        f=13
    end
    f()
end
""")

I've made changes in my local branch to do so, but there are still a few scoping problems, for e.g. when defining a constant inside a loop and changing it outside. I'm not sure if the problems are with what I did or some bug in the library... I'll make a pull request later