Serialization TODOs

[buybackoff]

• SM/ArrayConverter JSON returns exact arrays that are not powers of two, that causes problems in SM dispose/pool return. We use Read<TElement[]>, should try ArraySegment<TElement[]> and tweak impl to use pooled arrays if it doesn't already.
• Custom Utf8Json formatters to replace old Json.Net ones.
• Unhappy paths, e.g. small buffers, corrupted data in Read, etc.
• Separate header destination: if it's not null then it must be equal to current. Existing api could redirect to ptr and ptr + 4.