Prints around full target build regardless of replay or build
Printer indent: Use atomicModifyIORef' instead of modifyIORef
Refactor: Extract panicOnError
TODO
PriorityQueue: Support more than 2 priorities

(Still not in use)
Add failing test for bad parallelism/order
Priorities propagate to new dependent target slaves. Increased during building
Avoid parallelism release/acquire gap between parent and spawned slaves