Make ReadPos/Slice generic over Borrow and PosRead

[dekellum]

WIP, will summarize.

As of 7e02853, ReadPos (and ReadSlice) are now generic over a Barrow and PosRead type, in the form:

pub struct ReadPos<P, B>
where P: PosRead, B: Borrow<P>

This allows ReadPos/Slice to support, for example, an owned file (ReadPos<File, File>) as well as by shared reference (ReadPos<File, &File> or ReadPos<File, Arc<File>>. It also makes it possible for other (future) implementations of PosRead to share these types.

For all but the owned case, a type is needed for P which can not be inferred, e.g.

ReadPos::<File, _>::new(Arc::new(f), 10)

The existing rs::Read* types are left in place by type alias, e.g.:

pub type ReadSlice = super::ReadSlice<File, Arc<File>>;

This has been simplified, see update below.

A new ReadSubSlice trait must be imported to continue to use subslice. This is, strictly-speaking a breaking change.

[dekellum]

As of 58b1f26, PosRead has been implemented over all Borrow<File>, such that ReadPos (and ReadSlice) are now more simply generic over PosRead and have form:

pub struct ReadPos<P>
where P: PosRead

So these can own, for example via ReadPos<File> or use a shared reference, as in ReadPos<&File> or ReadPos<Arc<File>> and there is no type inference challenges.

The existing rc::Read* types are left in place by type alias, e.g.:

pub type ReadSlice = super::ReadSlice<Arc<File>>;

[dekellum]

Dropped ReadSubSlice trait, the only remaining breaking change for this, in 664a71c and merged in fc3a82d.