Remove the per-hasher digest type. Instead, store hash digests inside
the hashers and "borrow" it. In all cases, we're going to copy it into a
`Multihash<S>` anyways.

This:

1. Removes bunch of code.
2. Means that hashers don't need to be generic over the size (unless
they actually support multiple sizes). This fixes the UX issue
introduced in the const generics PR.
3. Avoids some copying.

BREAKING CHANGE

1. `Hasher.digest` no longer exists. Users should use
`Code::SomeCode.digest` where possible.
2. The hasher digests no longer exist.

The multihash digest may not fit.

fixes #141

Use config flags instead of introducing the new attribute `io_path`.

With some compile-time asserts we can make sure that certain `unwrap()`
calls won't panic.