Serialization optimizations for .NET Core 3.1

[ReubenBond]

String serialization:

|   Method |      Mean |  Gen 0 | Allocated |
|--------- |----------:|-------:|----------:|
|   Before |  94.83 ns | 0.0194 |     112 B |
|    After |  68.11 ns |      - |         - |

String deserialization:

|   Method |      Mean |  Gen 0 | Allocated |
|--------- |----------:|-------:|----------:|
|   Before | 134.18 ns | 0.0527 |     304 B |
|    After | 109.08 ns | 0.0334 |     192 B |

[sergeybykov]

It appears to break a bunch of test.

[ReubenBond]

Yep, I will take a look, maybe tomorrow

[ReubenBond]

Ok, it was an edge case - the output buffer needs to have some space or there'll be an exception.