Implement HIGH() and LOW() operators

They work with the 16-bit registers BC, DE and HL, returning the
corresponding 8-bit register. HIGH() works with AF as well, returning A.

They also work with any kind of constant or symbol, generating a RPN
patch in the object file if the value is not defined at assembly time.

They work with macro arguments as well.

Signed-off-by: Antonio Niño Díaz <antonio_nd@outlook.com>

src/asm/globlex.c

@@ -282,6 +282,9 @@ struct sLexInitString staticstrings[] = {
 	{"atan", T_OP_ATAN},
 	{"atan2", T_OP_ATAN2},
 
+	{"high", T_OP_HIGH},
+	{"low", T_OP_LOW},
+
 	{"strcmp", T_OP_STRCMP},
 	{"strin", T_OP_STRIN},
 	{"strsub", T_OP_STRSUB},