This enables for instance to start an SSH connection, and then
use various channels over it.

The SSHChannel no longer is a subclass of an SSHSession, which made
things awkward when attempting to use the session again.

Instead, the session is a field in the channel, and new channels can
be constructed from the session if this is available.

This also turns off the sample that defaulted to publicKey
authentication.

Session.fingerprintBytes, new methods that returns an array of bytes, rather than a formatted string

Add new authentication system: byCallback, to allow me to wire up the secure enclave in SwiftTermApp

Do not surface LibSSH2 to the swiftmodule, this fixes the 'no such module' error I get when consuming the library

* Swift 5 migration

* Migrated deprecated `withUnsafeBytes`

* Guard raw buffer pointer count before using it

Return error if it's empty

* Fixed whitespaces

#14