Limit the number of threads when using qemu to 1. Also, don't bother
running the stress test as this will trigger qemu bugs. Finally, also
make the stress test actually stress test.

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.

This commit is a significant overhaul of the library in an effort to head
towards a stable API. The rope implementation as well as a number of buffer
implementations have been removed from the library and will live at
https://github.com/carllerche/bytes-more while they incubate.

**Bytes / BytesMut**

`Bytes` is now an atomic ref counted byte slice. As it is contigous, it offers
a richer API than before.

`BytesMut` is a mutable variant. It is safe by ensuring that it is the only
handle to a given byte slice.

**AppendBuf -> ByteBuf**

`AppendBuf` has been replaced by `ByteBuf`. The API is not identical, but is
close enough to be considered a suitable replacement.

**Removed types**

The following types have been removed in favor of living in bytes-more

* RingBuf
* BlockBuf
* `Bytes` as a rope implementation
* ReadExt
* WriteExt