Sort out feature flags for everything in the engine mod

[nicklan]

We have many things protected by default/sync engine be enabled, but also arrow_conversion and arrow_expression, and they interact in complex ways.

There's probably some combination of flags that won't work.

Let's decide on a simple set of flags that compose well and switch to that.

[nicklan]

Also, update the code as suggested by #331 (comment) when this gets done