tag/Handler: pass StringView to OnTag() and OnPair()

Eliminates a number of allocations, because callers don't need to copy
the strings to a newly allocated buffer only to null-terminate them.
And most callers don't need to have a null-terminated string.

src/TagPrint.cxx

@@ -21,6 +21,7 @@
 #include "tag/Tag.hxx"
 #include "tag/Settings.hxx"
 #include "client/Response.hxx"
+#include "util/StringView.hxx"
 
 void
 tag_print_types(Response &r) noexcept
@@ -31,6 +32,13 @@ tag_print_types(Response &r) noexcept
 			r.Format("tagtype: %s\n", tag_item_names[i]);
 }
 
+void
+tag_print(Response &r, TagType type, StringView value) noexcept
+{
+	r.Format("%s: %.*s\n", tag_item_names[type],
+		 int(value.size), value.data);
+}
+
 void
 tag_print(Response &r, TagType type, const char *value) noexcept
 {