Allow compound types to be composed of type_id

This means that now the following code is valid
```
import scheme;
import mesh;
import math;

let m:mesh, m = cartesian1dMesh(-1,1,100);

let pi:R, pi = acos(-1);

let f: R^1 -> R, x -> x[0]*x[0];
let g: R^1 -> R, x -> sin(pi*x[0]);

let (fh,gh): Vh*Vh, (fh,gh) = (interpolate(m, P0(), f), interpolate(m, P0(), g));
```
This allows to define builtin functions that return composed
results. For instance the result of the calculation of a time step of
Lagrangian schemes. One can now return (rho,u,E,...).