Factor out FIFO state in block evaluators

[grebe]

In a current WiP change, I'm putting FIFO state in the register state map. This is convenient and avoids the need for API changes, but has some shortcomings.

Long term, it seems better to factor out FIFO state. Some benefits:

• Initializing registers is already clunky and the current approach makes it worse.
• Using std::deque or circular buffer is more performant.
• Factoring state out may make it easier to share FIFO implementation w/ the JIT.