Recycle space when reserving from Vec-backed Bytes (#197)

* Recycle space when reserving from Vec-backed Bytes

BytesMut::reserve, when called on a BytesMut instance which is backed by
a non-shared Vec<u8>, would previously just delegate to Vec::reserve
regardless of the current location in the buffer. If the Bytes is
actually the trailing component of a larger Vec, then the unused space
won't be recycled. In applications which continually move the pointer
forward to consume data as it comes in, this can cause the underlying
buffer to get extremely large.

This commit checks whether there's extra space at the start of the
backing Vec in this case, and reuses the unused space if possible
instead of allocating.

* Avoid excessive copying when reusing Vec space

Only reuse space in a Vec-backed Bytes when doing so would gain back
more than half of the current capacity. This avoids excessive copy
operations when a large buffer is almost (but not completely) full.