The `Source` trait was essentially covering the same case as `IntoBuf`,
so remove it.

While technically a breaking change, this should not have any impact due
to:

1) There are no reverse dependencies that currently depend on `bytes`
2) Source was not supposed to be implemented externally
3) IntoBuf provides the same implementations as `Source`

Given these points, the change should be safe to apply.

This change tracks the original capacity requested when `BytesMut` is
first created. This capacity is used when a `reserve` needs to allocate
due to the current view being too small. The newly allocated buffer will
be sized the same as the original allocation.

Add `?Sized` bounds to work for DST objects and also add impls for `Box` as well
as `&mut`

Enables collecting the contents of a `Buf` value into a relevant
concrete buffer implementation.

Allows links in other crates to link to crates.io docs of bytes itself.

Instead of providing a separate `try_reclaim` function, `reserve` will
attempt to reclaim the existing buffer before allocating.

The previous implementation didn't factor in a single `Bytes` handle
being stored in an `Arc`. This new implementation correctly impelments
both `Bytes` and `BytesMut` such that both are `Sync`.

The rewrite also increases the number of bytes that can be stored
inline.

* `drain_to` and `split_off` take &self and return Bytes.
* `drain_to_mut` and `split_off_mut` take &mut self and return BytesMut