std: add a Writer impl for &mut [u8]

[erickt]

@alexcrichton mentioned in the io reform rfc that there might be interest in reviving the Writer for &mut [u8], so I revived the PR for consideration. As written, it needs to do some unsafe magic in order to advance the mutable slice. It would be nice if this was someday factored out into a method on the slice itself.

[sfackler]

Can you add a cfail test to make sure that let mut v = vec![]; write!(v, "hi"); doesn't compile?

[Gankra]

@erickt status?

[nwin]

The unsafe part can actually be reduced to (the transmute is just there for the lifetime)

*self = unsafe { mem::transmute(self.slice_from_mut(write_len)) };

I still wonder why it is needed for the Writer but not for the Reader.

[huonw]

r? @aturon

[alexcrichton]

Closing due to inactivity, but feel free to reopen once more of the I/O reform RFCs have landed!