async; ValueTask vs Task

Odd findings.txt

Objective: compare Task<T> vs ValueTask<T> in a scenario that should
be ideal for ValueTask<T> - non-trivial results, but usually (always)
already completed.

Hypothesis: ValueTask<T> should be more efficient, as it does not (in
the "already completed" case) involve allocation.

Methodology: create a test rig that computes a number using multiple
nested roll-up operations, comparing sync vs async-Task<T> vs async-ValueTask<T>;
use "async"/"await" for the rollups in the async cases. Run all tests using
BenchmarkDotNet from command-line in release mode. It is expected that both
Task<T> and ValueTask<T> will have some overhead compared to non-async version.

Follow-up: added "HandCranked" versions that seek to avoid the await machinery.
Faster, but ugly as sin.

Code: https://github.com/mgravell/protobuf-net/tree/async/src/TheAwaitingGame

Results: ValueTask<T> appears to take *longer* than Task<T>; update - but it allocates far less:

--------------------------------------------

```
> dotnet run -c Release -f net462

Total time: 00:03:57 (237.7 sec)

// * Summary *

BenchmarkDotNet=v0.10.3.111-nightly, OS=Windows 10.0.14393
Processor=Intel(R) Core(TM) i7-5930K CPU 3.50GHz, ProcessorCount=12
Frequency=3416975 Hz, Resolution=292.6565 ns, Timer=TSC
dotnet cli version=1.0.3
  [Host]     : .NET Core 4.6.25009.03, 64bit RyuJIT
  Job-LKSOIG : .NET Core 4.6.25009.03, 64bit RyuJIT

Force=False

                         Method |       Mean |    StdErr |    StdDev |     Op/s |  Gen 0 | Allocated |
------------------------------- |-----------:|----------:|----------:|---------:|-------:|----------:|
                           Sync | 10.9782 us | 0.0168 us | 0.0580 us | 91089.95 |      - |      0 kB |
                      TaskAsync | 17.7219 us | 0.0532 us | 0.1919 us |  56427.3 | 3.8625 |  24.25 kB |
               TaskCheckedAsync | 17.5696 us | 0.0480 us | 0.1732 us | 56916.64 | 3.8625 |  24.25 kB |
                 ValueTaskAsync | 21.5139 us | 0.0526 us | 0.1895 us | 46481.58 |      - |      0 kB |
          ValueTaskWrappedAsync | 20.4348 us | 0.0396 us | 0.1371 us | 48936.02 | 0.6333 |   3.99 kB |
 ValueTaskDecimalReferenceAsync | 17.4158 us | 0.0562 us | 0.2025 us | 57419.19 | 1.5458 |    9.7 kB |
          ValueTaskCheckedAsync | 18.8079 us | 0.0203 us | 0.0732 us | 53169.22 |      - |      0 kB |
               HandCrankedAsync | 15.1644 us | 0.0283 us | 0.1020 us | 65944.09 |      - |      0 kB |
           AssertCompletedAsync | 16.3290 us | 0.1084 us | 0.4057 us | 61240.56 |      - |      0 kB |
                TaskDoubleAsync | 15.2432 us | 0.0418 us | 0.1506 us | 65603.14 | 3.4750 |  21.82 kB |
           ValueTaskDoubleAsync | 17.1229 us | 0.1955 us | 0.8059 us | 58401.46 |      - |      0 kB |
```

--------------------------------------------

```
> dotnet run -c Release -f netcoreapp1.1

Total time: 00:01:23 (83.08 sec)

// * Summary *

BenchmarkDotNet=v0.10.3.0, OS=Microsoft Windows 10.0.15063
Processor=Intel(R) Core(TM) i7-7700HQ CPU 2.80GHz, ProcessorCount=8
Frequency=2742190 Hz, Resolution=364.6720 ns, Timer=TSC
dotnet cli version=1.0.2
  [Host]     : .NET Core 4.6.25009.03, 64bit RyuJIT
  DefaultJob : .NET Core 4.6.25009.03, 64bit RyuJIT


           Method |      Mean |    StdDev |     Gen 0 | Allocated |
----------------- |---------- |---------- |---------- |---------- |
             Sync | 3.0868 ms | 0.0634 ms |         - |      68 B |
        TaskAsync | 4.7674 ms | 0.0915 ms | 1796.8750 |   6.06 MB |
   ValueTaskAsync | 5.7727 ms | 0.0591 ms |         - |     135 B |
 HandCrankedAsync | 4.6280 ms | 0.0449 ms |         - |     135 B |
 ```