c#: Use Stack based return pointer for imports

[jsturtevant]

This updates the imports path to use a stack based return pointer similar to the way rust does it.

There are a few considerations:

• the stack allocations are defined by the return payload so could potentially get larger than expected. Since its the programmer making this decision in the API I wasn't overly concerned. This could be mitigated by checking the size and allocating it on the heap as we are today
• we have to do a slight over allocation with the c# stackalloc function since there is no way to control the alignment

I ran some iterations around the larger VariantCasts variant import test case and saw a slight speed difference (mostly negligible) and a ~13-15% reduction in memory usage:

# stack based
Average Time: 6.686999999999999 µs
Average Memory: 35463.2 bytes
Average Allocated Bytes: 31085.48 bytes

# heap based 
Average Time: 6.752000000000001 µs
Average Memory: 41055.52 bytes
Average Allocated Bytes: 36881.76 bytes