Avoid module qualification in the interface of the io module where possible.

library/io.m:
	Avoid module qualification in the interface where possible.
	I've left the module qualifiers on the types io.state/0 and io.error/0
	since they just look strange without them.  (We don't really use io.state/0
	much these days anyway.)

	Most the result types (e.g. io.res/0) still require module
	qualification in order to distinguish them from the result types
	in the stream module.

	s/input_stream/text_input_stream/ for the all uses of the type
	input_stream.  Similarly for output streams.  This was what was
	intended when the equivalences were originally introduced and the
	debugger has used the text_* names for many years now.

library/store.m:
	s/io\.state/io/ in a few spots.