Oid is defined as unsigned 32. On some Python implementations (probably
the ones where maxint = 2 ** 31) this can cause int overflow for large
values (see #961). On my 64 box it doesn't seem the case.

Oid handling was sloppy here and there (messages, casts...): trying to
use uint everywhere, and added a couple of helper macros to treat Oid
consistently.

Close #961.

Close #836.

Now its state is unmodified, so apart from special-casing creation
and initial population can work unmodified, and all the desired
properties just work (modifiability, picklability...)

Close #886.