Fix chain remaining_mut(), allowing to chain growing buffer

[Zettroke]

Stumbled upon panic when was trying to chain fixed buffer with growing buffer.

Fixed remaining_mut for chain, so you can chain growing buffer.

There is workaround for this, so I am not sure if this is intended or not.

let mut buff = [0u8; 128];
let vec = vec![];
let chained = (&mut buff[..]).chain_mut(vec.limit(usize::MAX - buff.len()));

But I decided to fix it, because I think this workaround is inconvenient.

Only potential problem is:
Calls to remaining_mut can return same value (usize::MAX) before and after write to buffer.

let mut buff = [0u8; 128];
let vec = vec![];
let mut chained = (&mut buff[..]).chain_mut(&mut vec);
let prev_remaining = chained.remaining_mut();
chained.put_slice(b"hi");
assert_eq!(prev_remaining, chained.remaining_mut());

[taiki-e]

Thanks for the PR and sorry for the late review!

I tend to favor this, but, I would like to hear others' opinions.

cc @carllerche @seanmonstar @Darksonn: Do you think it's ok to allow remaining_mut to return less value than it actually is? (in the cases it is greater than the usize::MAX, or in all cases?)

• There is currently no documentation about it.
• This is not allowed in the upper bound of the iterator's size_hint. (Add ExactSizeIterator impl for iter::Chain rust-lang/rust#66531)

[Zettroke]

Due to #501 I am no longer sure if this PR still useful. To cause overflow now you will need to chain fixed buffer with 2 vectors(or Vec with 2 more), which is very unlikely.

[Darksonn]

I think it is fine to allow remaining_mut to be less than the actual capacity.