doc/dev: add corpus.rst

Structure of the corpus dir, how to add to it.

Signed-off-by: Sage Weil <sage@inktank.com>
cls_refcount: add type tests

Signed-off-by: Sage Weil <sage@inktank.com>
cls_rgw: add missing type tests

Signed-off-by: Sage Weil <sage@inktank.com>