Add `uninit` intrinsic. Use in `core::vec`

[brson]

vec previously had some code that looked like this:

let vp: *T = ...;
let v = move *vp;

This had the effect of memcpying *vp to v and zeroing *vp. move is being removed so this doesn't work. I've replaced that with:

let mut vp: *T = ...;
let mut v: T = rusti::init(); // Create a zeroed value
v <-> *vp;

This could probably be more efficient by 1) not zeroing the value (in most cases), 2) not doing the swap.

Probably this is what we should do:

let mut vp: *T = ...;
let mut v: T = rusti::uninit(); // Create a non-zeroed value
ptr:memcpy(&mut T, vp, 1);

And if you need the zeroing you can do a memset.

[graydon]

non-critical for 0.6, de-milestoning

[sanxiyn]

This seems to be a duplicate of #3471.

[Aatch]

Closed by #6354