Handle combine

[timcassell]

• Combined MakeReady and Handle while still allowing the stack to unwind. Potential performance improvements #26
• Reduced size of Promise(<T>)s when T is a small struct.
• Corrected async Promise continuation to execute on the same thread as the last awaited object.
• Refactored unit tests to be more reliable with improperly used objects.
• Updated Progress to always schedule the next waiter on the same context, even after progress has already completed.
• Optimized All/Merge/Race/First promises to remove a field to reduce memory consumption, and no longer schedule on the execution scheduler.

Current master:

|         Type | Pending | Recursion |       Mean | Allocated | Survived |
|------------- |-------- |---------- |-----------:|----------:|---------:|
|   AsyncAwait |    True |         1 |   4.242 μs |         - |    248 B |
|   AsyncAwait |    True |        10 |  43.471 μs |         - |  2,480 B |
|   AsyncAwait |    True |       100 | 429.178 μs |         - | 24,800 B |
| ContinueWith |    True |         1 |   3.861 μs |         - |     80 B |
| ContinueWith |    True |        10 |  38.541 μs |         - |    800 B |
| ContinueWith |    True |       100 | 386.543 μs |         - |  8,000 B |

This PR:

|         Type | Pending | Recursion |       Mean | Allocated | Survived |
|------------- |-------- |---------- |-----------:|----------:|---------:|
|   AsyncAwait |    True |         1 |   4.127 us |         - |    224 B |
|   AsyncAwait |    True |        10 |  41.251 us |         - |  2,240 B |
|   AsyncAwait |    True |       100 | 409.602 us |         - | 22,400 B |
| ContinueWith |    True |         1 |   3.516 us |         - |    160 B |
| ContinueWith |    True |        10 |  35.678 us |         - |    880 B |
| ContinueWith |    True |       100 | 351.374 us |         - |  8,080 B |