Added NUMBER_OF_OPCODES which aids in compilation errors

If I add a new operand I want the build system to be more helpful in
finding the places I need to change to make it work.

src/inst.h

@@ -65,6 +65,8 @@ typedef enum
   OP_EQ_HWORD,
   OP_EQ_WORD,
 
+  // Should not be an opcode
+  NUMBER_OF_OPCODES,
   OP_HALT = 0b11111111, // top of the byte is a HALT
 } opcode_t;