Treat raw pointers with more respect

[hawkw]

I think there are at least a few cases where we can work on refactoring code that uses raw pointers to be somewhat more idiomatic:

• where can raw pointers be replaced with ptr::Unique<T>s?
  • We already use Unique in some cases
  • I think there might be more uses of raw pointers that can be replaced with Unique.
• where can pointer math be replaced with pointer.offset()?
  • this is considered more idiomatic
  • has less ugly casts/can be less wordy
• where can coercing raw pointers to references be replaced with pointer.as_ref() or pointer.as_mut()?
  • these functions can still return references to uninitialised memory, but they're more null-safe (allow us to handle null pointers Optionally)
  • fewer ugly casts/transmutes
• where can mem::transmute-ting a reference into a raw pointer be replaced with &*?
  • this is more idiomatic

[hawkw]

Another way we could potentially improve safety is by 'downgrading' some *mut pointers to *const pointers. Potentially, there might also be some *const pointers that could be downgraded to &'static references...