reparametrize_function_with_proto: Function reparametrization
In cezden/ExpFamilyLab: Abstraction of the Exponential Families

Creates a function being a reparametrisation (with simplification) of f1 * g, e.g. f1(g(x))

1	reparametrize_function_with_proto(f1, g, res.proto)

`f1`	function 1
`g`	function 2
`res.proto`	resulting function prototype with NULL body, e.g. `function(theta) NULL`

 reparametrize_function_with_proto(
   function(x) x^2,
   function(y) y^3,
   function(z) NULL
   )
   # function(z) z^6

 reparametrize_function_with_proto(
   function(x) x^2 + 2*x,
   function(y) (y+1)^3,
   function(z) NULL
   )
   # function(z) ((1 + z)^3 + 2) * (1 + z)^3