Lint against suspicious upvalue overwriting in loop

[Kampfkarren]

for _, thing in things do
    values[key] = thing
end

...where key is an upvalue and unmodified, and there's no differing branches (like a continue that would make this potentially a "last that isn't filtered"). Hit a bug like this recently.

[Kampfkarren]

We should probably limit this to otherwise pure bodies

[chriscerie]

This can probably extend to anything where the upvalue being overwritten is the same, like a regular variable

In the help message perhaps suggest using # for arrays and next for dictionaries, if that was the user intention