This patch fixes the `copy_to_slice` function, rectifying the logic.
However, the incorrect code does not result in incorrect behavior as the
only case `cnt != src.len()` is during the final iteration, and since
`src.len()` is greater than `cnt` in that case, `off` will be
incremented by too much, but this will still trigger the `off <
dst.len()` condition.

The only danger is `src.len()` could cause an overflow.

* make Buf and BufMut usable as trait objects

- All the `get_*` and `put_*` methods that take `T: ByteOrder` have
  a `where Self: Sized` bound added, so that they are only usable from
  sized types. It was impossible to make `Buf` or `BufMut` into trait
  objects before, so this change doesn't break anyone.
- Add `get_n_be`/`get_n_le`/`put_n_be`/`put_n_le` methods that can be
  used on trait objects.
- Deprecate the export of `ByteOrder` and methods generic on it.

* remove deprecated ByteOrder methods

Removes the `_be` suffix from all methods, implying that the default
people should use is network endian.

- All the `get_*` and `put_*` methods that take `T: ByteOrder` have
  a `where Self: Sized` bound added, so that they are only usable from
  sized types. It was impossible to make `Buf` or `BufMut` into trait
  objects before, so this change doesn't break anyone.
- Add `get_n_be`/`get_n_le`/`put_n_be`/`put_n_le` methods that can be
  used on trait objects.
- Deprecate the export of `ByteOrder` and methods generic on it.

Fixes #163 

Update to match master version of IoVec (0.2.0?), using
IoVec/IoVecMut instead of &IoVec and &mut IoVec.

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.

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