Support abstract consts, ban redefining consts in all circumstances

[fredemmott]

This is currently fine:

class A {
  const FOO = 'bar';
}

class B extends A {
  const FOO = 'baz';
}

Ideally, this would just be forbidden - however, static::BAR is used in some patterns. One way to ban this in the general case would be to support abstract constants.

[milesj]

+1

[lexidor]

We've got abstract consts now.
Redeclaring them is not banned though.

abstract class A {
  abstract const string CONSTANT;
}

class B extends A {
  const string CONSTANT = 'B';
}

class C extends B {
  const string CONSTANT = 'C';
}