adds MacroPlugin.pluginsIsBlackbox

This is an important omission in the current macro plugin API, which was
designed before the blackbox vs whitebox separation was implemented.

Even if one overrides pluginsTypedMacroBody and pluginsMacroExpand,
that would still be not enough to write a custom macro expander, because
typedImplicit1 uses isBlackbox, which is tightly coupled with the standard
way of reading/writing macro signatures.