lazy value stack

Values are now computed as soon as an expression succeeds, even if that value will later be discarded. Since a parse is inherently a tree, a stack should be able to compute the full derivation. Thus we could have a value stack with (start, end, function) triples. This means that a new tuple is created for each expression that emits values, but that is already the case as the emitted values `args` is an immutable tuple and not a list. The difference then is that the function is not computed until it is needed. Since there are now callables for Capture and Bind, all features can be represented this way (although in some tests, Capture and Bind were a bit slower than the `~` and `:` operators).