Allow single-character DCI strings

The DCI string format uses character values where the high bit of the
last byte differs from the rest of the string.  Usually all the high
bits are clear except on the last byte, but SourceGen generally allows
either polarity.

This gets a little uncertain with single-character strings, because
SourceGen can't auto-detect DCI very effectively.  A series of bytes
with the high bit set could be a single high-ASCII string or a series
of single-byte DCI strings.

The motivation for allowing them is C64 PETSCII.  While ASCII allows
"high ASCII" as an escape hatch, PETSCII doesn't have that option, so
there's no way to mark the data as a character or a string.  We still
want to do a bit of screening, but if the user specifies a non-ASCII
character set and the selected bytes have their high bits set, we
want to just treat the whole set as 1-byte DCI.

Some minor adjustments were needed for a couple of validity checks
that expected longer strings.

This adds some short DCI strings in different character sets to the
char-encoding regression tests.

(for issue #102)

SourceGen/SGTestData/Source/20120-char-encoding.S

@@ -1,7 +1,9 @@
 ; Copyright 2019 faddenSoft. All Rights Reserved.
 ; See the LICENSE.txt file for distribution terms (Apache 2.0).
 ;
-; Assembler: ACME (has good PETSCII/screen code support)
+; Assembler: ACME v0.96.4 (has good PETSCII/screen code support)
+; (acme -o 20120-char-encoding 20120-char-encoding.S)
+;
 ; 6502 version
 ;
 ; This becomes multiple tests that differ only in how the default
@@ -145,8 +147,10 @@ allbytes
         !hex    f0f1f2f3f4f5f6f7f8f9fafbfcfdfeff
 
 end     nop
-        jmp     skip_bs
+        jmp     skip_esc
 
+; String literals with backslash escapes.  Necessary because ACME v0.97
+; introduced the feature and broke compatibility with previous versions.
         !text   "ASCII escapes \: \\ \r \n \",$22," \",$27," \t"
         !byte   $80
         !pet    "PETSCII escapes \: \\ \r \n \",$22," \",$27," \t"
@@ -159,5 +163,46 @@ end     nop
         }
         !byte   $80
 
-skip_bs nop
+skip_esc nop
+        jmp     skip_dci
+
+; Small batches of DCI strings, for testing single-character strings.
+; EDIT: format all of these as DCI strings
+        !byte   $86
+
+        !text   "Hell",$ef                      ;Hello
+        !text   "H",$c9                         ;Hi
+        !byte   $d8                             ;X
+        !byte   $a1                             ;!
+        !byte   $86
+
+        !xor $80 {
+            !text   "Hell",$ef                      ;Hello
+            !text   "H",$c9                         ;Hi
+            !byte   $d8                             ;X
+            !byte   $a1                             ;!
+        }
+        !byte   $86
+
+        !pet    "hellO"
+        !pet    "hI"
+        !byte   $d8
+        !byte   $a1
+        !byte   $86
+
+        !xor $80 {              ;hi then lo; probably not necessary
+            !pet    "hellO"     ;(all C64 DCI seems to be lo-then-hi)
+            !pet    "hI"
+            !byte   $d8
+            !byte   $a1
+        }
+        !byte   $86
+
+        !scr    "Hell",$8f      ;just do lo-then-hi for screen codes
+        !scr    "H",$c9
+        !byte   $d8
+        !byte   $a1
+        !byte   $86
+
+skip_dci nop
         rts