streaming.c: move {open,close,read} from vtable to "struct git_istream"

streaming.c: move {open,close,read} from vtable to "struct git_istream"

Move the definition of the structure around the open/close/read
functions introduced in 46bf043807c (streaming: a new API to read from
the object store, 2011-05-11) to instead populate "close" and "read"
members in the "struct git_istream".

This gets us rid of an extra pointer deference, and I think makes more
sense. The "close" and "read" functions are the primary interface to
the stream itself.

Let's also populate a "open" callback in the same struct. That's now
used by open_istream() after istream_source() decides what "open"
function should be used. This isn't needed to get rid of the
"stream_vtbl" variables, but makes sense for consistency.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>