cgen.py: use C type classes for _CodeGen._ft_c_type()

cgen.py: use C type classes for _CodeGen._ft_c_type()

This patch introduces new classes to represent C types in `cgen.py`.

The C type class hierarchy is:

    _CType
      _ArithCType
      _PointerCType

A `_PointerCType` object contains another `_CType` object.

You can get the string representation of a C type object with its
__str__() method.

A C type constructor's `is_const` parameter controls whether or not the
_value_ is const. For example:

`const char *`:
    `_PointerCType` containing a const `_ArithCType`.

`const char * const`:
    Const `_PointerCType` containing a const `_ArithCType`.

_CodeGen._ft_c_type() now returns a `_CType` object instead of a string.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>