Fix most warnings

All the complier complains about now are the missing docs and one odd warning on test, which I cannot figure out right now (see below).

```
/Users/ilya/Code/rust/zinc/src/zinc/util/volatile_cell.rs:30:7: 35:4 warning: method is never used: `new`, #[warn(dead_code)] on by default
/Users/ilya/Code/rust/zinc/src/zinc/util/volatile_cell.rs:30   pub fn new(value: T) -> VolatileCell<T> {
/Users/ilya/Code/rust/zinc/src/zinc/util/volatile_cell.rs:31     VolatileCell {
/Users/ilya/Code/rust/zinc/src/zinc/util/volatile_cell.rs:32       value: value,
/Users/ilya/Code/rust/zinc/src/zinc/util/volatile_cell.rs:33       invariant: marker::InvariantType::<T>,
/Users/ilya/Code/rust/zinc/src/zinc/util/volatile_cell.rs:34     }
/Users/ilya/Code/rust/zinc/src/zinc/util/volatile_cell.rs:35   }
```

Not sure what's up with that...