Typechecker: state variables declared as constant

[bitbart]

Extend the typechecker so that it detects when a state variable declared as constant may be assigned somewhere in the contract other than in its declaration. For example, the contract:

contract C {
    int constant N=1;
    constructor() { N=2; } 
    function f(int n) external { }
  }

should not pass typechecking, because N is assigned in the constructor.

Additionally, check that constants are always initialized. For example, the contract:

contract C {
    int constant N;
    constructor() { } 
    function f(int n) external { }
  }

should not pass type checking.