Implement hybrid push/pull semantics

[milibopp]

See this paper by Conal Elliott for reference. The general idea to see cells as a sequence piecewise continuous functions of time makes sense.

Contrary to Elliott, time will not be handled explicitly but rather implicitly by the sequence of transactions. Also the implementation itself will be inherently imperative, as the purely functional, lazy evaluation approach is not feasible in Rust (too much boilerplate to emulate Haskell features).