Instead of using the `marl::Event` synchronization primitives, drop all the way down to using `marl::Scheduler::Fiber` APIs directly, and use the new `marl::Scheduler::Fiber::wait()` function that does not take a lock.

Provides around another 2x performance boost:

```
go run ./third_party/marl/tools/cmd/benchdiff/main.go pre.json post.json
Delta                | Test name                                | (A) pre.json | (B) post.json
-2.08x -81.186µs     | Coroutines/Fibonacci/iterations:512      | 156.202µs    | 75.016µs
-2.10x -668.201µs    | Coroutines/Fibonacci/iterations:4096     | 1.275069ms   | 606.868µs
-2.11x -5.359326ms   | Coroutines/Fibonacci/iterations:32768    | 10.166126ms  | 4.8068ms
-2.12x -10.342µs     | Coroutines/Fibonacci/iterations:64       | 19.585µs     | 9.243µs
-2.13x -2.784542745s | Coroutines/Fibonacci/iterations:16777216 | 5.251299045s | 2.4667563s
-2.13x -347.220746ms | Coroutines/Fibonacci/iterations:2097152  | 653.812928ms | 306.592182ms
-2.14x -43.615678ms  | Coroutines/Fibonacci/iterations:262144   | 82.017312ms  | 38.401634ms
```

Bug: b/145754674
Change-Id: I5f6b0c8c92af645cc2a825c6f1e2769b2440638e
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/42850Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>