Rewrote the ledger in test_bytes_vec_alloc.rs to not piss off miri. The ledger is now a table within the allocator, which seems to satisfy miri. The old solution was to bundle an extra usize into the beginning of each allocation and then index past the start when deallocating data to get the size.
The `bytes()` / `bytes_mut()` name implies the method returns the full
set of bytes represented by `Buf`/`BufMut`. To rectify this, the methods
are renamed to `chunk()` and `chunk_mut()` to reflect the partial nature
of the returned byte slice.
`bytes_vectored()` is renamed `chunks_vectored()`.
Closes#447
