Ensure relational expressions are treated as values.
Remove oldies.
Fix compiler strategy on leaf operands.
Fix spec_helper -> compiler_helper
Add a Plan abstraction to avoid compiling same expressions multiple times.
Remove that horrible `bind` stuff.
Update the protocol for obtaining base cogs.
Move compiler selection code to Plan.
Add support for compilers joining the current plan.
Refactor Plan/SubPlan
Avoid reusing compilation results; unsound.