Pipe operator should propagate eagerly

[obuchtala]

ATM there is an optimization which stops propagation of expression evaluation if the value is the same as before. But this is problematic as some subsequent expressions might need to be updated, even if the input does not change (e.g., randon generator, etc.)

In Stencila this has lead to stencila/stencila#279, where a plot did not get rerendered, because the data did not change.