Split out the core and main modules.

- Implicitly import everything in core into every imported module.
- Test cyclic imports.

include/wren.h

@@ -109,8 +109,13 @@ void wrenFreeVM(WrenVM* vm);
 // Runs [source], a string of Wren source code in a new fiber in [vm].
 //
 // [sourcePath] is a string describing where [source] was located, for use in
-// debugging stack traces. It must not be `null`. If an empty string, it will
-// not be shown in a stack trace.
+// debugging stack traces. It must not be `null`.
+//
+// If it's an empty string, then [source] is considered part of the "core"
+// module. Any module-level names defined in it will be implicitly imported by
+// another other modules. This also means runtime errors in its code will be
+// omitted from stack traces (to avoid confusing users with core library
+// implementation details).
 WrenInterpretResult wrenInterpret(WrenVM* vm, const char* sourcePath,
                                   const char* source);