Begun work on bignums; changed integer size to 64 bits

I'm fairly sure the size of a long int on my machines is 64 bit anyway, but for portability it needs to be explicit.

src/arith/integer.c

@@ -35,7 +35,7 @@ long double numeric_value( struct cons_pointer pointer ) {
 /**
  * Allocate an integer cell representing this value and return a cons pointer to it.
  */
-struct cons_pointer make_integer( long int value ) {
+struct cons_pointer make_integer( int64_t value ) {
     struct cons_pointer result = allocate_cell( INTEGERTAG );
     struct cons_space_object *cell = &pointer2cell( result );
     cell->payload.integer.value = value;