function_argument_walker should start from innermost, not outermost argument call chain

[dcbaker]

if you have something like

files(files(files(files('foo')))

It currently takes four loops to resolve this, because the evaluation order is from outside to inside (the simplest solution). However, by simply resolving from innermost files call first, we can avoid 3 iterations of the optimizer, since we can lower the whole thing to [File{'foo'}] on the first pass