twn_util.h: separate internal things away, remove indirect includes in places

docs/interop.txt

@@ -4,6 +4,7 @@ for that certain steps are taken:
 * number of public api calls is kept at the minimum
 * procedure signatures can only use basic types, no aggregates, with exception of Vec/Matrix types and alike,
   with no expectation on new additions (see /include/twn_types.h)
+* optionals can be expressed via pointer passage of value primitives, with NULL expressive default
 * /include/twn_game_api.json file is hand-kept with a schema to aid automatic generation and other tooling
 
 one of main inspirations for that is opengl model