Use deterministic randomness fuzzing the pooling allocator (#5247)

This commit updates the index allocation performed in the pooling
allocator with a few refactorings:

* With `cfg(fuzzing)` a deterministic rng is now used to improve
  reproducibility of fuzz test cases.
* The `Mutex` was pushed inside of `IndexAllocator`, renamed from
  `PoolingAllocationState`.
* Randomness is now always done through a `SmallRng` stored in the
  `IndexAllocator` instead of using `thread_rng`.
* The `is_empty` method has been removed in favor of an `Option`-based
  return on `alloc`.

This refactoring is additionally intended to encapsulate more
implementation details of `IndexAllocator` to more easily allow for
alternate implementations in the future such as lock-free approaches
(possibly).

crates/runtime/Cargo.toml

@@ -21,7 +21,7 @@ memoffset = "0.6.0"
 indexmap = "1.0.2"
 thiserror = "1.0.4"
 cfg-if = "1.0"
-rand = "0.8.3"
+rand = { version = "0.8.3", features = ['small_rng'] }
 anyhow = { workspace = true }
 memfd = "0.6.1"
 paste = "1.0.3"