Revert stop_character_mask to a complement.

Somehow this makes the MSVC build faster, so we want it.

This time without any more warnings (afaik), by appending
the "u" unsigned suffix to the hex mask for type compatibility.

Author: jpetso

cppcodec/detail/stream_codec.hpp

@@ -241,7 +241,7 @@ struct alphabet_index_info
     static constexpr const alphabet_index_t padding_idx = 1 << 8;
     static constexpr const alphabet_index_t invalid_idx = 1 << 9;
     static constexpr const alphabet_index_t eof_idx = 1 << 10;
-    static constexpr const alphabet_index_t stop_character_mask = 0xFF00;
+    static constexpr const alphabet_index_t stop_character_mask{~0xFFu};
 
     static constexpr const bool padding_allowed = padding_searcher<
             CodecVariant, num_possible_symbols>::exists_padding_symbol();