Remove inner `mut` on vectors

This patch removes syntax of the form:
```
let foo = [mut 1,2,3,4];
```
in favor of:
```
let mut foo = [1,2,3,4];
````
All parts of the compiler have been converted, except for libcore (hit a strange ICE), librustc, and the compile-fail tests (figured those shouldn't change, and should get new tests if they need them).