Marl now has a new flag on the task to say that it should be run on the same thread that calls `marl::schedule()`.

This can dramatically reduce the scheduling overhead when bouncing between two tasks, as other threads do not have to be woken.

```
> go run ./third_party/marl/tools/cmd/benchdiff/main.go pre.json post.json
Delta                    | Test name                                | (A) pre.json    | (B) post.json
-15.53x -1m16.299750679s | Coroutines/Fibonacci/iterations:16777216 | 1m21.551049724s | 5.251299045s
-15.71x -1.206771937s    | Coroutines/Fibonacci/iterations:262144   | 1.288789249s    | 82.017312ms
-15.72x -9.624071378s    | Coroutines/Fibonacci/iterations:2097152  | 10.277884306s   | 653.812928ms
-15.77x -35.755µs        | Coroutines/Fibonacci/iterations:8        | 38.176µs        | 2.421µs
-15.77x -150.190662ms    | Coroutines/Fibonacci/iterations:32768    | 160.356788ms    | 10.166126ms
-15.79x -18.864275ms     | Coroutines/Fibonacci/iterations:4096     | 20.139344ms     | 1.275069ms
-15.93x -2.332202ms      | Coroutines/Fibonacci/iterations:512      | 2.488404ms      | 156.202µs
-15.96x -292.896µs       | Coroutines/Fibonacci/iterations:64       | 312.481µs       | 19.585µs
```

Bug: b/145754674
Change-Id: I0e014083e1dbc9f5cdf51e7abc378df6be22d805
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/42410Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>