all: Simplify mp_int_t/mp_uint_t definition.

Assuming proper C99 language support, we can select "the int type as big as
a pointer" (most of the time) or "the 64-bit int type" (nanboxing with
REPR_D), and then define everything else automatically.

This simplifies port configuration files.  And the types can still be
overridden if needed.

Signed-off-by: Jeff Epler <jepler@unpythonic.net>

docs/develop/porting.rst

@@ -161,8 +161,6 @@ The following is an example of an ``mpconfigport.h`` file:
 
    // Type definitions for the specific machine.
 
-   typedef intptr_t mp_int_t; // must be pointer size
-   typedef uintptr_t mp_uint_t; // must be pointer size
    typedef long mp_off_t;
 
    // We need to provide a declaration/definition of alloca().