PDD 22 - I/O release candidate 1

[Allison Randal]

I've committed an updated I/O PDD. I'm close to pronouncing this ready 
to implement, so get in your comments now.

One piece that is currently missing is a discussion of which lightweight 
concurrency model we're going to use for the asynchronous operations. 
I've had ongoing back-channel conversations with various people, but I 
need to congeal them. Pitch in your own 2 cents.

Also, any reactions to the distinction that async ops return status 
objects while sync ops return integer error codes? Sync opcodes could 
have 2 signatures, one with an integer return type (int error code) and 
one with a PMC return type (status object).

Allison