Analyzer Plugins

AnnotationCheckers are insufficient because they live outside the
compiler cake and it's not possible to pass a Typer into an annotation
checker.

Analyzer plugins hook into important places of the compiler:

  - when the namer assigns a type to a symbol (plus a special hook for
    accessors)
  - before typing a tree, to modify the expected type
  - after typing a tree, to modify the type assigned to the tree

Analyzer plugins and annotation checker can be activated only during
selected phases of the compiler.

Refactored the CPS plugin to use an analyzer plugin (since
adaptToAnnotations is now part of analyzer plugins, no longer
annotation checkers).