`lexeme` should accept an arbitrary skipper as its argument

[saki7]

Currently, when we want to alter the skipper and invoke lexeme at the same time, we must write this pattern:

constexpr auto p = skip(blank)[lexeme[some_parser]];

If we could write this way, it would be much simpler and also more understandable:

constexpr auto p = lexeme(blank)[some_parser];

Implementor's note: blank and space are currently specially treated as a builtin skipper, so the lexeme directive cannot (and should not) produce their temporary instance; we need to use the builtin handling in lexeme also.