Array QoL methods

[yui-915]

Introduces new helper methods to make some common patterns less verbose.

• xs.items.add(i) -> xs.at(i)

  • returns *mut T instead of T to allow assignment like *xs.at(i) = x;.
  • couldn't find a better name (get would imply returning a value).
  • doesn't do any bounds checking (read comment).
  • using it in generic da_* function requires adding a Copy constraint to T ,
    but this should be fine as crust™ requires all types to be Clone + Copy.

• for i in 0..xs.count { let x = *xs.at(i); /*...*/ } -> for x in xs.iter() { /*...*/ }

  • for iterating over values (T).

• for i in 0..xs.count { let x = xs.at(i); /*...*/ } -> for x in xs.iter_mut() { /*...*/ }

  • for iterating over pointers (*mut T, can implicitly convert to *const T).

• existing functions like da_last and da_last_mut can be turned into methods but I kept them as-is for backwards compatibility (da_append would be tricky to get working as a method without references).

• The last commit is more or less an excuse for me to use Array::iter() more, feel free to drop it if you feel it's unnecessary.