Add binary operator '_' for left shift

If the command line option yus to allow the underscore character in
symbols is NOT selected, then interpret underscore as a new binary
operator meaning to do a left shift of the left side value by the
number of bit positions indicated by the right side value.  As for the
arithmetic operators, both values must be literal (numeric constants
or symbols equated to a numeric constant).

tests/test-prec.mac

@@ -20,6 +20,11 @@
 	.word	12. / 4		; 000003
 	.word	123 & 771	; 000121
 	.word	123121 ! 322	; 123323
+five = 5
+a:	.word	123 _ 3	 	; 001230
+	.word	five _ 6	; 000500
+	.word	101_five	; 004040
+	.word	a _ 3		; Invalid expression
 
 	.word	- 377		; 177401
 	.word	+ 377		; 000377