blackbox restriction #3: can't affect implicit search

When an application of a blackbox macro is used as an implicit candidate,
no expansion is performed until the macro is selected as the result of
the implicit search.

This makes it impossible to dynamically calculate availability of
implicit macros.
blackbox restriction #4: can't customize pattern matching

When an application of a blackbox macro is used as an extractor in a
pattern match, it triggers an unconditional compiler error,
preventing customizations of pattern matching implemented with macros.