Assignment to immutable and constant variables

[bitbart]

Extend the interpreter so that it reverts a transaction whenever a state variable with mutability declared as constant is assigned.

For example, consider the contract:

contract C {
    int constant N=1;
    int x;
    function f(int n) external { if (n>0) x+=1; else N=0; }
  }

A call to f(0) should revert, since it would trigger an assignment to the constant N.

State variables declared as immutable can be assigned, but only in the constructor. For example, in the contract:

contract C {
    int immutable x;
    constructor(int n) { x = n; }
    function f(int n) external { if (n==0) x=0; }
  }

The assignment in the constructor does not revert, but a call to f(0) should revert, since it would trigger an assignment to the immutable variable x.