Reduce async overhead

[timcassell]

Followup to #1941 (fixes #1595 and fixes #1738)

Refactored delegates to reduce async measurement overhead (sync measurements are the same).

Also added IterationSetup/Cleanup async support.

Master:

|                      Method |          Mean |      Error |     StdDev |  Gen 0 | Allocated |
|---------------------------- |--------------:|-----------:|-----------:|-------:|----------:|
|          AsyncYieldTaskVoid | 1,777.6317 ns | 11.4813 ns | 10.1779 ns | 0.0381 |     120 B |
|      AsyncImmediateTaskVoid |     2.3197 ns |  0.0294 ns |  0.0246 ns |      - |         - |
|     AsyncYieldValueTaskVoid | 1,852.0409 ns |  2.9352 ns |  2.7456 ns | 0.0381 |     120 B |
| AsyncImmediateValueTaskVoid |    15.3422 ns |  0.1608 ns |  0.1505 ns |      - |         - |
|                    SyncVoid |     0.1591 ns |  0.0042 ns |  0.0037 ns |      - |         - |
|           AsyncYieldTaskInt | 1,850.0632 ns | 13.8105 ns | 12.2426 ns | 0.0381 |     120 B |
|       AsyncImmediateTaskInt |    39.0781 ns |  0.2674 ns |  0.2371 ns | 0.0229 |      72 B |
|      AsyncYieldValueTaskInt | 1,889.4795 ns | 19.5146 ns | 18.2539 ns | 0.0401 |     128 B |
|  AsyncImmediateValueTaskInt |    41.2959 ns |  0.1417 ns |  0.1183 ns |      - |         - |
|                     SyncInt |     0.3043 ns |  0.0089 ns |  0.0083 ns |      - |         - |

This PR:

|                      Method |        Job | UnrollFactor |          Mean |      Error |      StdDev |  Gen 0 | Allocated |
|---------------------------- |----------- |------------- |--------------:|-----------:|------------:|-------:|----------:|
|                    SyncVoid | DefaultJob |           16 |     0.1627 ns |  0.0058 ns |   0.0051 ns |      - |         - |
|                     SyncInt | DefaultJob |           16 |     0.3143 ns |  0.0363 ns |   0.0322 ns |      - |         - |
|          AsyncYieldTaskVoid | Job-CXHPYP |            1 | 1,461.6091 ns | 80.2756 ns | 235.4344 ns | 0.0381 |     120 B |
|      AsyncImmediateTaskVoid | Job-CXHPYP |            1 |     4.4395 ns |  0.0255 ns |   0.0238 ns |      - |         - |
|     AsyncYieldValueTaskVoid | Job-CXHPYP |            1 | 1,543.3745 ns | 30.5159 ns |  57.3162 ns | 0.0381 |     120 B |
| AsyncImmediateValueTaskVoid | Job-CXHPYP |            1 |    11.1546 ns |  0.0417 ns |   0.0390 ns |      - |         - |
|           AsyncYieldTaskInt | Job-CXHPYP |            1 | 1,597.0157 ns | 15.7257 ns |  13.9404 ns | 0.0381 |     120 B |
|       AsyncImmediateTaskInt | Job-CXHPYP |            1 |    43.2262 ns |  0.1489 ns |   0.1393 ns | 0.0229 |      72 B |
|      AsyncYieldValueTaskInt | Job-CXHPYP |            1 | 1,576.6878 ns | 30.8910 ns |  53.2854 ns | 0.0401 |     128 B |
|  AsyncImmediateValueTaskInt | Job-CXHPYP |            1 |    33.3715 ns |  0.2657 ns |   0.2356 ns |      - |         - |

public class Benchmark
{
    public long counter;

    [Benchmark]
    public async Task AsyncYieldTaskVoid()
    {
        await Task.Yield();
        unchecked { ++counter; }
    }

    [Benchmark]
    public Task AsyncImmediateTaskVoid()
    {
        unchecked { ++counter; }
        return Task.FromResult(true);
    }

    [Benchmark]
    public async ValueTask AsyncYieldValueTaskVoid()
    {
        await Task.Yield();
        unchecked { ++counter; }
    }

    [Benchmark]
    public ValueTask AsyncImmediateValueTaskVoid()
    {
        unchecked { ++counter; }
        return new ValueTask();
    }

    [Benchmark]
    public void SyncVoid()
    {
        unchecked { ++counter; }
    }

    [Benchmark]
    public async Task<long> AsyncYieldTaskInt()
    {
        await Task.Yield();
        unchecked { return ++counter; }
    }

    [Benchmark]
    public async Task<long> AsyncImmediateTaskInt()
    {
        unchecked { return ++counter; }
    }

    [Benchmark]
    public async ValueTask<long> AsyncYieldValueTaskInt()
    {
        await Task.Yield();
        unchecked { return ++counter; }
    }

    [Benchmark]
    public async ValueTask<long> AsyncImmediateValueTaskInt()
    {
        unchecked { return ++counter; }
    }

    [Benchmark]
    public long SyncInt()
    {
        unchecked { return ++counter; }
    }
}

If you want, we can close #1941 in favor of this, but there are more changes here which should be reviewed. Or #1941 can be merged before this, which is also fine.

Merging this PR will also make it easier/possible to add async engine support in the future.

cc @AndreyAkinshin @adamsitnik @stephentoub