Use stdint.h for int64_t / uint64_t in API.

Now that we can use stdint.h, we can use it to portably define
64 bit integer types for use in the API.

src/api/api_stats.cpp

@@ -130,7 +130,7 @@ extern "C" {
         Z3_CATCH_RETURN(0.0);
     }
 
-    __uint64 Z3_API Z3_get_estimated_alloc_size(void) {
+    uint64_t Z3_API Z3_get_estimated_alloc_size(void) {
         return memory::get_allocation_size();
     }