Join `Stream' and `Connection'

The `Stream' and the `Connection' modules/concepts has been quite
similar and, in some ways, overlapping. The `Stream' and `Connection'
modules allow for direct access to the XML stream. The `Stream' module
exported an API for opening, restarting, and closing the stream. The
`Connection' module, on the other hand, allowed for pushing and
pulling elements and stanzas to and from the stream. Furthermore, the
`Connection' type was used in both the `Connection' module and the
higher-level `Stream' module.

This patch joins the two modules into one `Stream' module, and renames
the `Connection' type to `Stream'. It also renames most other
connection-related functions and types. Also, `connect' is renamed
`openStream' and `closeStreams' is renamed `closeStream' (the stream
is `bidirectional' in RFC 6120 terminology).