New class hierarchy for Skylark functions

* New hierarchy BaseFunction > UserModeFunction, BuiltinFunction.
  The old hierarchy still exists for now, to be deleted after migration:
  Function > AbstractFunction > MixedModeFunction >
    (UserModeFunction, SkylarkFunction > SimpleSkylarkFunction)
  (UserModeFunction is already migrated, and
  BaseFunction implements Function, for now.)

* Function supports *args and **kwargs when calling functions, and
  mandatory named-only parameters in the style of Python 3.
  Notable difference with Python: *args binds the variable to a tuple,
  because a Skylark list would have to be monomorphic.

* A better, simpler, safer FFI using reflection with BuiltinFunction.
  Handles typechecking, passes parameters in a more Java style.
  (Not used for now, will be used later.)

* A new annotation @SkylarkSignature, intended to replace @SkylarkBuiltin,
  supports the full function call protocol, including default arguments.

* Support for annotating function Factory-s rather than functions.

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MOS_MIGRATED_REVID=88958581