Rework CFunction arguments treatment

It now uses FunctionArgumentConverter and ASTNodeDataTypeFlattener. This leads
to a more consistent implementation and use of CFunction along with
FunctionExpression.

Now one can write
``
import math;
let x_and_3: R->R*R, x->(x,3);
R x = pow(x_and_3(2));
``
the novelty is that `pow` acts (has one could expect) as a `R*R->R` function
expression which makes is more natural (even if this example may not show its
relevance)