core::vec is missing methods for mutable slices

[june0cho]

Fix #5984. Also, I found a problem on type inference and left a comment.

[brson]

@june0cho Thanks! Can you file an issue about the method inference problem?

[nikomatsakis]

I don't believe the failure to find a method is a bug, but rather the impl definitions in the PR are not quite right. In particular, the mut_slice method is defined as:

 +    fn mut_slice(&mut self, start: uint, end: uint) -> &'self mut [T] {...}

but the receiver type is &mut [T]. Really the mut_slice method should therefore just take self, I should think. (Once dynamically sized types lands, the current self type would be the correct one, because the impl type would be [T] not &mut [T]).

[june0cho]

@nikomatsakis you are right. I corrected as you commented and the error does not occur anymore.
I'll update it !