Add method to extend an iopipe fully

[schveiguy]

One may want a buffer of all the data an iopipe can read. The easiest thing to do is this:

while(iop.extend(0) != 0) {}

There should be a function that does this, similar to ensureElems.

[schveiguy]

Hm... ensureElems(size_t.max) would probably work too... Nevermind.