Enable MemoryDiagnoser on Legacy Mono

[MichalPetryka]

This enables MemoryDiagnoser on Legacy Mono (since it provides GetAllocatedBytesForCurrentThread) and makes it more resilient to errors.

[timcassell]

@MichalPetryka Is this good to go? It's still in draft.

[MichalPetryka]

Is this good to go?

I wanted to check with a test benchmark and forgot about it, it seems to work fine:

| Method | Job                | Runtime            | Mean      | Error     | StdDev    | Gen0   | Allocated |
|------- |------------------- |------------------- |----------:|----------:|----------:|-------:|----------:|
| Alloc  | .NET 8.0           | .NET 8.0           | 11.401 ns | 1.2076 ns | 3.5605 ns | 0.0255 |      40 B |
| Alloc  | .NET Framework 4.8 | .NET Framework 4.8 |  6.179 ns | 0.6341 ns | 1.8597 ns | 0.0255 |      40 B |
| Alloc  | Mono               | Mono               | 16.793 ns | 1.1951 ns | 3.5238 ns | 0.0115 |      48 B |
| Alloc  | Mono with .NET 8.0 | Mono with .NET 8.0 | 18.935 ns | 0.3219 ns | 0.3832 ns | 0.0115 |      48 B |

[timcassell]

Yep, I tested it with my own benchmarks also, and it all seems good to me. Thanks @MichalPetryka!