utf8.h: Add a #define synonym

This is more clearly named for various uses in this file.  It has an
unwieldy length, but is unlikely to be used outside it.

utf8.h

@@ -252,7 +252,7 @@ are in the character. */
  * sequence contributes to the specification of the code point.  In the bit
  * maps above, you see that the first 2 bits are a constant '10', leaving 6 of
  * real information */
-#define UTF_ACCUMULATION_SHIFT		6
+#  define UTF_CONTINUATION_BYTE_INFO_BITS 6
 
 /* ^? is defined to be DEL on ASCII systems.  See the definition of toCTRL()
  * for more */
@@ -269,10 +269,16 @@ are in the character. */
 
 #endif /* EBCDIC vs ASCII */
 
-/* 2**UTF_ACCUMULATION_SHIFT - 1.  This masks out all but the bits that carry
+/* Since the significant bits in a continuation byte are stored in the
+ * least-significant positions, we often find ourselves shifting by that
+ * amount.  This is a clearer name in such situations */
+#define UTF_ACCUMULATION_SHIFT  UTF_CONTINUATION_BYTE_INFO_BITS
+
+/* 2**info_bits - 1.  This masks out all but the bits that carry
  * real information in a continuation byte.  This turns out to be 0x3F in
  * UTF-8, 0x1F in UTF-EBCDIC. */
-#define UTF_CONTINUATION_MASK  ((U8) (nBIT_MASK(UTF_ACCUMULATION_SHIFT)))
+#define UTF_CONTINUATION_MASK                                               \
+                        ((U8) nBIT_MASK(UTF_CONTINUATION_BYTE_INFO_BITS))
 
 /* For use in UTF8_IS_CONTINUATION().  This turns out to be 0xC0 in UTF-8,
  * E0 in UTF-EBCDIC */

utfebcdic.h

@@ -207,8 +207,7 @@ explicitly forbidden, and the shortest possible encoding should always be used
  * there are 6 bits of real information in a UTF-8 continuation byte vs. 5 bits
  * in a UTF-EBCDIC one. */
 
-#define UTF_ACCUMULATION_SHIFT		5
-
+#define UTF_CONTINUATION_BYTE_INFO_BITS  5
 /* Also needed is how perl handles a start byte of 8 one bits.  The decision
  * was made to just append the minimal number of bytes after that so that code
  * points up to 64 bits wide could be represented.  In UTF-8, that was an extra