c#: perfer using fixed over GCalloc

[jsturtevant]

see:

• c#: Use span and memory apis for primitive type parameters on imports #1138 (comment)
• c#: Handle Cabi realloc post return  #1145 (comment)
• c#: Use span and memory apis for primitive type parameters on imports #1138 (comment)
• c#: Use span and memory apis for primitive type parameters on imports #1138 (comment)

[pavelsavara]

It is then "best practice" to keep stack allocations small (all stackallocs for a single method should typically add up to not more than 1024 bytes) and to never make them "dynamic" in length (instead rounding up to the largest buffer size).

From dotnet/runtime#52065 (comment)