String literal support

[alexoltean61]

This PR adds basic support for string literals, implementing #264. Each string literal is desugared to the application of a new function, which stores chunks in memory and returns their starting pointer.

In the test cases, I used this to define high-level revert and assert functions similar to regular Solidity.

Please note that string literals in pattern matching equations are still not supported. For example, this code snippet will not compile (but it does not compile on the main branch either):

import dispatch;

contract Match {
    function matchStr() -> () {
        match "test" {
            | "test" => return ();
            | _ => return ();
        }
    }
}