Simplify the OP_POP_* routine by adjusting data_type_t

This simple fix made the routine for OP_POP not require an additional
dispatch step on top of the conditional due to OPCODE_DATA_TYPE,
instead using data_type_t as a map from an opcode's base type to a
specific type.

lib/base.h

@@ -72,7 +72,7 @@ typedef union
  */
 typedef enum
 {
-  DATA_TYPE_NIL = 0,
+  DATA_TYPE_NIL = -1,
   DATA_TYPE_BYTE,
   DATA_TYPE_HWORD,
   DATA_TYPE_WORD,