refactor(kernel): split ipc/memory/lib/thread by concern; trim comments

crates/kernel/src/tests/captree_model.rs

@@ -1,8 +1,4 @@
-//! Randomized check of the derivation tree against an independent model.
-//! The model applies the rules of docs/ipc.md (derive records a parent,
-//! delete re-parents, revoke removes the subtree, death deletes everything
-//! a process holds) to its own data; after every step the kernel must hold
-//! exactly the capabilities the model says are alive.
+//! Randomized check of the derivation tree against a model of the rules in docs/ipc.md.
 
 extern crate std;