[feature] Add Tail Call Optimization (TCO)

[lattiahirvio]

Add TCO. TCO would be rather easy to implement (the standard way to do it is reusing the stack frame on tail calls), and it should add no additional complexity for the end user. TCO optimizes recursive calls, and thus helps with things like recursive descent parsers, tree walking, and other activities that are traditionally done recursively.

[JoeStrout]

To be clear, this does not optimize recursion in general. It only optimizes the special case where you are doing return plus a call to the same function, e.g.:

factorial = function(n, accum=1)
    if n <= 1 then return accum
    return factorial(n-1, accum*n)
end function

If the return is anything other than just the recursive call, TCO does not apply. So the more common form of factorial:

factorial = function(n)
    if n <= 1 then return 1
    return n * factorial(n-1)
end function

could not benefit from this optimization.

[lattiahirvio]

Correct.