String literals

[mbenke]

Idea:

import dispatch;
// function f() -> memory(string) { return "uint256"; }) should desugar to

function stringLit75696e74323536() -> memory(string) {
        let size : word = 7;
	let ptr = allocate_memory(64);
        assembly {
            mstore(ptr, size)
            mstore(add(ptr,32), 0x75696e7432353600000000000000000000000000000000000000000000000000)
        }
	return memory(ptr);
}

function f() -> memory(string) {
  return stringLit75696e74323536();
}

with appropriate chunking for longer strings.

Parts of #176 can be reused.