ABI: uint8 too small for potential complex quad itemsize bits?

[seberg]

If quad-precision takes off, then the current uint8 for the itemsize cannot represent the 128+128 = 256 bits of a complex quad number. Even now that would fail for most long double storage formats (those cannot be represented right now, so I am not sure it matters).

If an ABI break is necessary in the future, maybe this should be bumped to uint16 (maybe some other ones as well)? I guess there will be other ways to work around the limitation, though. And I admit quad-precision complex may well be as bad as it gets and is just at the limit.

[leofang]

We should list a few items that would be great to do if we plan to break ABI.