Rollup merge of #59979 - stepancheg:num-size, r=ehuss

to_xe_bytes for isize and usize returns an array of different size

... on different platforms.

Official rustdoc of
[`usize::to_le_bytes`](https://doc.rust-lang.org/std/primitive.usize.html#method.to_le_bytes)
displays signature

```
pub fn to_ne_bytes(self) -> [u8; 8]
```

which might be misleading: this function returns 4 bytes on 32-bit
systems.

With this commit applied rustdoc for `isize` and `usize` is this:
<img width="740" alt="2019-04-15_0020" src="https://user-images.githubusercontent.com/28969/56100765-9f69b380-5f14-11e9-974c-daa25edaa881.png">

0 files changed, 0 insertions, 0 deletions