Boot core library from image rather than source

This should speed up start time and reduce malloc/free
usage to about 15% of what is what previously for startup.
The current cost is slightly larger binary as the representaion
of the image is currently less compact than source code.

src/core/fiber.c

@@ -433,5 +433,5 @@ static const JanetReg fiber_cfuns[] = {
 
 /* Module entry point */
 void janet_lib_fiber(JanetTable *env) {
-    janet_cfuns(env, NULL, fiber_cfuns);
+    janet_core_cfuns(env, NULL, fiber_cfuns);
 }