Fix over-long bitshift on 32-bit hosts.

Fixes #4. (Hopefully.)

src/interpreter.rs

@@ -1001,7 +1001,7 @@ fn make_aggregate_repr<V>(&self, variant_fields: V) -> Repr
             variants.push(fields);
         }
 
-        let discr_size = match variants.len() {
+        let discr_size = match variants.len() as u64 {
             n if n <= 1       => 0,
             n if n <= 1 << 8  => 1,
             n if n <= 1 << 16 => 2,