Removing some mutable fields and unsafe blocks

This removes some of the easier instances of mutable fields where the explicit self can just become `&mut self` along with removing some unsafe blocks which aren't necessary any more now that purity is gone.

Most of #4568 is done, except for [one case](https://github.com/alexcrichton/rust/blob/less-mut-fields/src/libcore/vec.rs#L1754) where it looks like it has to do with it being a `const` vector. Removing the unsafe block yields:

```
/Users/alex/code/rust2/src/libcore/vec.rs:1755:12: 1755:16 error: illegal borrow unless pure: creating immutable alias to const vec content
/Users/alex/code/rust2/src/libcore/vec.rs:1755         for self.each |e| {
                                                           ^~~~
/Users/alex/code/rust2/src/libcore/vec.rs:1757:8: 1757:9 note: impure due to access to impure function
/Users/alex/code/rust2/src/libcore/vec.rs:1757         }
                                                       ^
error: aborting due to previous error
```

I also didn't delve too much into removing mutable fields with `Cell` or `transmute` and friends.