Add sandboxing API.

The sandboxing API is meant to make janet a bit more attractive
for certain application embedding use cases. The sandboxing API
puts limits on what system resources the interpreter can access.

src/core/state.h

@@ -135,6 +135,9 @@ struct JanetVM {
     size_t scratch_cap;
     size_t scratch_len;
 
+    /* Sandbox flags */
+    uint32_t sandbox_flags;
+
     /* Random number generator */
     JanetRNG rng;