Add tasty.reflect.Kernel

[nicolasstucki]

The idea is to move all logic from the TASTy reflect API that needs to be implemented by the compiler to the kernel. This will allow us to decouple the user-facing API from the compiler implementation.

[liufengyun]

LGTM. Well done 👍

This is a nice step towards the split between user-facing contracts & compiler-facing contracts. Some comments for further improvements (for later PRs, this PR is already huge):

1. Better naming scheme. Names are always a headache for programming, currently the method name def ClassSymbol_companionClass is not nice.

2. Kernel is now the interface between macro system & compiler, thus safe construction of trees should not be a design goal for Kernel, as the APIs are not used by programmers . A worthwhile design goal would be to minimize the contract & defend against compiler changes. For example, Kernel does not need to define the same set of abstract types as Core. In terms of minimizing interface, currently it's not ideal:

  def Tree_pos(self: Tree)(implicit ctx: Context): Position
  def Term_pos(self: Term)(implicit ctx: Context): Position
  def Pattern_pos(self: Pattern)(implicit ctx: Context): Position
  def TypeTree_pos(self: TypeTree)(implicit ctx: Context): Position
  def Id_pos(self: Id)(implicit ctx: Context): Position

In the design of Kernel, can we assume that Scala2 is going to implement it as well some day? That may help with the design.

[liufengyun]

It's better to find another name for Core, to avoid conflicts with Kernel.

[nicolasstucki]

What about ReflectionTypes?

[nicolasstucki]

Rebased

[liufengyun]

This PR reduces compile time for the Dotty project.