Small-features-21Q1 fix: Allow generic functions as bounds

[eernstg]

The small-features-21Q1 spec includes the feature described in #496, except that it does not mention generic function types used as bounds. I think we should include bounds: There is a need to specify a generic function type as a bound if a particular type variable is intended to have a value which is a generic function type, e.g., in order to know how many type arguments to pass in invocations.

class C<F extends void Function<X>(X)> {
  F f;
  C(this.f);
  void m() { f<int>(42); }
}

class D<F extends void Function<X>(X, [List<X>?]) extends C<X> {...}

Also, this PR fixes a typo tripple-shift --> triple-shift.

[eernstg]

@lrhn, do you wish to omit the bounds from #496?

[lrhn]

No, I want to allow all types everywhere, as arguments and as bounds.

It gets problematic otherwise: foo<F extends Function, G extends F>(){} with foo<T Function<T>(T), Never Function<T>(Object?)>() would either be a run-time type error because the bound is a generic function type, or it would always be a compile-time error to use a type variable as bound unless it cannot possibly be bound to a generic function type (so not an unbounded type variable, and not one bounded by Function, Object, FutureOr<Object>, or similar supertypes of generic function types).

[lrhn]

LGTM, thanks.