The public api is changed to require a context object for all
operations. Because other library objects store the context pointer
internally, only the constructor functions need an explicit context
object as a parameter.
The devinfo and clock event data is now cached internally at the device layer.
This allows the new dc_parser_new() convenience function to retrieve the event
data directly from the device handle, and applications don't have to deal with
the events anymore to create a parser.
Adding the "dc_" namespace prefix (which is of course an abbreviation
for libdivecomputer) should avoid conflicts with other libraries. For
the time being, only the high-level device and parser layers are
changed.
