WIP: Bug: Split out MemIO from IOStream

Discussion reference: https://groups.google.com/d/topic/julia-dev/4bDcs9A5614/discussion

Something like this is what I think a MemIO type could look like.  

However, this patch is horribly BROKEN: when building sys.ji, julia suddenly decides she wants more memory than my machine has, and everything freezes until the OOM killer kicks in.  From my changes, it is not at all obvious to me why.  Help!

Cheers,  Kevin

(I'll likely be unresponsive for the rest of the weekend...  Happy Labor Day!)