Introduced instructions to engage with a call stack

Essentially you may "call" an absolute program address, which pushes
the current address onto the call stack.  CALL_STACK does the same
thing but the absolute program address is taken from the data stack.
RET pops an address off the call stack then jumps to that address.

lib/inst.h

@@ -148,6 +148,10 @@ typedef enum
   OP_JUMP_IF_BYTE,
   OP_JUMP_IF_HWORD,
   OP_JUMP_IF_WORD,
+  // Subroutines
+  OP_CALL,
+  OP_CALL_STACK,
+  OP_RET,
 
   // Should not be an opcode
   NUMBER_OF_OPCODES,
@@ -231,6 +235,9 @@ inst_t *insts_read_bytecode_file(FILE *, size_t *);
 #define INST_JUMP_STACK ((inst_t){.opcode = OP_JUMP_STACK})
 #define INST_JUMP_IF(TYPE, OP) \
   ((inst_t){.opcode = OP_JUMP_IF_##TYPE, .operand = DWORD(OP)})
+#define INST_CALL(OP)   ((inst_t){.opcode = OP_CALL, .operand = DWORD(OP)})
+#define INST_CALL_STACK ((inst_t){.opcode = OP_CALL_STACK})
+#define INST_RET        ((inst_t){.opcode = OP_RET})
 
 #define INST_PRINT(TYPE) ((inst_t){.opcode = OP_PRINT_##TYPE})
 #endif