util/sha1: rework _mesa_sha1_{init,final}

util/sha1: rework _mesa_sha1_{init,final}

Rather than having an extra memory allocation [that we currently do not
and act accordingly] just make the API take an pointer to a stack
allocated instance.

This and follow-up steps will effectively make the _mesa_sha1_foo simple
define/inlines around their SHA1 counterparts.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Grazvydas Ignotas <notasas@gmail.com>