Pure modifier semantics

[bitbart]

Revise the semantics of TinySol so that functions declared as pure cannot read the state variables of the contract:

https://docs.soliditylang.org/en/latest/contracts.html#state-mutability

For example, in the contract:

contract C {
      uint x;
      function f() public pure returns(uint) { return (x+1); }
      function g() public { x = this.f(); }
}

any call to g must revert, since the function f reads the state variable x, and so it is not pure.