streaming.c: avoid forward declarations

streaming.c: avoid forward declarations

Change code added in 46bf043807c (streaming: a new API to read from
the object store, 2011-05-11) to avoid forward declarations of the
functions it uses. We can instead move this code to the bottom of the
file, and thus avoid the open_method_decl() calls.

Aside from the addition of the "static helpers[...]" comment being
added here, and the removal of the forward declarations this is a
move-only change.

The style of the added "static helpers[...]"  comment isn't in line
with our usual coding style, but is consistent with several other
comments used in this file, so let's use that style consistently here.

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