data: change alphabet::Gap integral type representation

The representation '*' was shared with NodeWildcard. I believe '*'
is better for NodeWildcard so let's change Gap to '~'.
At first I wanted to use '_' but it is already occupied by
BottomOfTheStackSymbol and the underscore is probably better suited for
that.

alib2data/src/alphabet/Gap.h

@@ -82,7 +82,7 @@ public:
 template < typename Base >
 inline Base Gap::instance ( ) {
 	if constexpr ( std::is_integral_v < Base > ) {
-		return '*';
+		return '~';
 	} else if constexpr ( std::is_same_v < Base, std::string > ) {
 		return std::string ( 1, Gap::instance < char > ( ) );
 	} else if constexpr ( std::is_same_v < Base, Gap > ) {