Automate number-to-peano conversion.

Automate the conversion of numbers from decimal to peano when passed as parameters to the relations "add" and "multiply".

Example:

``` python
goal = Goal.with_variables(
lambda x, y:
multiply(x, y, peano.to_peano(24)))
```

Will become:

``` python
goal = Goal.with_variables(
lambda x, y:
multiply(x, y, 24))
#24 is automatically converted to peano inside the relation.
```