TPU-friendly Historian

[garious]

Before these patches, the Historian owned both its input and output channels. If other stages in the TPU pipeline followed suit, there'd be no way to stitch them together. With these patches, we look to set a more composable precedent. We have 3 choices of which channel to hoist: the input, the output, or both. With a bit of experimentation, it seems hoisting just the input channel results in both the most natural code and the most flexible. One might guess that hoisting both would be more flexible, but because Sync leaks into the sender's type signature, the only advantage to hoisting the output channel is to set the size of it (so might as well just pass in an integer!)

A pipeline can now be written as:

let (input, output) = channel();
let a = Stage0::new(output);
let b = Stage1::new(a.output);
let c = Stage2::new(b.output);
input.send(42);
drop(input);
assert_eq(c.output.iter().collect().len(), 1)