bytes: Separate Bytes and BytesMut

[frankmcsherry]

This PR separates Bytes into Bytes and BytesMut. The former implements clone() and is unwriteable. The latter does not implement clone but is writeable. One goes from the latter to the former with the freeze() method.

The intent is to minimize the use of BytesMut, which is narrowed only to moments within the writing to a BytesSlab. As soon as the writing completes it yields a Bytes which is from that point on unwriteable.

One of the goals is to make serialized messages cloneable, so that broadcast channels can more efficiently be implemented.

[frankmcsherry]

The changes to bytes/src/lib.rs are a bit silly to read as a diff. As the functionality was split between the two, the diff inserts a type definition halfway through an impl block. It can also be read as "copy Bytes to BytesMut; make Bytes.ptr a const ptr; delete methods that do not get used".

[antiguru]

I think this looks good. I left some comments inline, but the overall structure makes sense to me.

You might want to add #[inline] annotations to simple functions (freeze, deref*) so we don't have to go back in the future should we notice that they're not inlined. As they don't have generic parameters, without LTO, they wouldn't be inlined.