Implements ClassVar introduced in PEP 526. Resolves #2771, #2879.

* Support annotating class attributes with ClassVar[...].
* Prohibit ClassVar annotations outside class bodies (regular variable
  annotations, function and method signatures).
* Prohibit assignments to class variables accessed from instances (including
  assignments on self).
* Add checks for overriding class variables with instance variables and vice
  versa when subclassing.
* Prohibit ClassVars nested inside other types.
* Add is_classvar flag to Var.
* Add nesting_level attribute to TypeAnalyser and use it inside helper methods
  anal_type and anal_array. Move analyzing implicit TupleTypes from
  SemanticAnalyzer to TypeAnalyser.
* Analyze ClassVar[T] as T and bare ClassVar as Any.
* Prohibit generic ClassVars and accessing generic instance variables via class
  (see #2878 comments).
* Add types.get_type_vars helper, which returns all type variables present in
  analyzed type, similar to TypeAnalyser.get_type_var_names.