String literals

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.

Start by reading issue #176, which the proposal identifies as reusable prior art, then trace how the compiler currently represents and lowers string literals. The work is done when string literals desugar to generated memory-returning functions and longer strings are chunked appropriately, with compiler tests covering the transformation.

Written by the indexing model from the issue text.