Support nested statements

[Knut Andreas Ruud]

Nested statements could be very useful to specify a pre-transformation to a clip inline, without having to set up a separate clip/cell with this transformation first. For instance, consider the difference between

[A1] beat
[A2] =A1 resize .5
[A3] =A1 A2 interleave -ranges 1/16

and

[A1] beat
[A2] =A1 (A1 resize .5) interleave -ranges 1/16