Tag macro: remaining work

[neko-kai]

• replace parameters in HKT: [T: Tag] => Either[T, ?] this means adding a macro case to HKTag and appropriate logic in TagMacro [i.e. extracting the real param from { type Arg }. done
• replace parameters in structural types: { def x: T ; type Y = G }. Keep generating weak type tags for structural part, in runtime, mutate the symbols inside MemberScope of a weak tag holding the structural with .setInfo – set their types to resolved parameters from Tag
• Replace parameters in type bounds _ >: G <: T. ( & existentials? steal code from shapeless)
• Make TagKUBound work. In macro, replace kindOf and Kind type with a more sophisticated check, also preserving type bounds so that we can summon the appropriate instance together with type bounds in summonTag

related #369 #350

[neko-kai]

Addendum: Replace parameters in type projections: def x[A <: X: Tag] => A#Component, including when prefix is a singleton: x[a.type] => a.Component // if a is a static object, not a free term

[pshirshov]

Having TagKK[F[A, B]] should provide us TagK[F[Something, ?]]

[neko-kai]

HKTag resolving is fixed. The other enhancements are split into new issues:

#771
#772
#773