Make ContainmentTree fully generic (vercel/turborepo#4620)

### Description

This turns the ContainmentTree struct into a fully generic data
structure. The previous PR in this stack already made the chunks values
generic, but this goes one step further by making the key type generic
as well.

### Testing Instructions

Unit test

Author: alexkirsz

crates/turbopack-core/Cargo.toml

@@ -36,6 +36,7 @@ turbo-tasks-build = { workspace = true }
 
 [dev-dependencies]
 rstest = { workspace = true }
+tokio = { workspace = true }
 
 [features]
 default = []