implement brush cost gauges

they're a little ugly at the moment, and can be a little useless for most simple brushes, but whatever we'll make them better later

crates/haku/src/vm.rs

@@ -73,10 +73,18 @@ impl Vm {
         }
     }
 
+    pub fn num_refs(&self) -> usize {
+        self.refs.len()
+    }
+
     pub fn remaining_fuel(&self) -> usize {
         self.fuel
     }
 
+    pub fn remaining_memory(&self) -> usize {
+        self.memory
+    }
+
     pub fn set_fuel(&mut self, fuel: usize) {
         self.fuel = fuel;
     }