Fix tail calls in to be closed context

[arnodel]

Although it is not documented, the Lua 5.4 test suite checks that when returning from a function at a point when there are pending to-be-closed variables, tail-calls are disabled so that the closing action can be performed after the call.

e.g.

function f()
    local x <close> = mkclose()
    return g() -- This would normally be a tail-call
end

This PR implements this restriction.

[cderici]

Yeah this makes sense. I initially thought that the close functions were in tail position with respect to the returning function, but I guess the close functions can close over the variables (that are passed by reference types like tables/fucntions) from the caller like

function f()
    t = {1,2}
    local x <close> = mkclose(t)
    local y <close> = mkclose(t)
    return g() -- This would normally be a tail-call
end

so f's frame in the stack needs to stay where it is, meaning they can't guarantee proper tail calls when there's to-be-closed vars, especially when the close functions are just arbitrary code (i.e. can overflow).