Type the TokenKind consts correctly.

You can write a surprising amount of code where the compiler will shrug
and silently coerce things for you.  Right up until you can't.
(Some test cases that'll be coming down the commit queue shortly
happened to end up checking the type of the constants, and, well.
Suddenly this was noticable.)

codec/codectools/token.go

@@ -56,17 +56,17 @@ func (tk Token) String() string {
 type TokenKind uint8
 
 const (
-	TokenKind_MapOpen   = '{'
-	TokenKind_MapClose  = '}'
-	TokenKind_ListOpen  = '['
-	TokenKind_ListClose = ']'
-	TokenKind_Null      = '0'
-	TokenKind_Bool      = 'b'
-	TokenKind_Int       = 'i'
-	TokenKind_Float     = 'f'
-	TokenKind_String    = 's'
-	TokenKind_Bytes     = 'x'
-	TokenKind_Link      = '/'
+	TokenKind_MapOpen   TokenKind = '{'
+	TokenKind_MapClose  TokenKind = '}'
+	TokenKind_ListOpen  TokenKind = '['
+	TokenKind_ListClose TokenKind = ']'
+	TokenKind_Null      TokenKind = '0'
+	TokenKind_Bool      TokenKind = 'b'
+	TokenKind_Int       TokenKind = 'i'
+	TokenKind_Float     TokenKind = 'f'
+	TokenKind_String    TokenKind = 's'
+	TokenKind_Bytes     TokenKind = 'x'
+	TokenKind_Link      TokenKind = '/'
 )
 
 type ErrMalformedTokenSequence struct {