more docs, remove Ceiling / Priority / Level traits

Cargo.toml

README.md

build.rs

src/lib.rs

tests/cfail/borrow.rs

tests/cfail/ceiling.rs

tests/cfail/lock.rs

tests/cfail/race-1.rs

tests/cfail/race-2.rs

tests/cfail/tasks-p0.rs

tests/cfail/tasks-same-handler.rs

tests/cfail/tasks-wrong-idle.rs

tests/cfail/tasks-wrong-init.rs

tests/cfail/tasks-wrong-priority.rs

tests/cfail/tasks-wrong-task.rs