memoryview assignment fails for .NET numeric types

The following should work:
```py
import System
m = memoryview(bytearray(1))
m[0] = System.Byte()
```

Appears to be a regression from IronPython 2. Causes `socket.recvfrom_into(memoryview)` to fail. The C# implementation of `recvfrom_into` should probably be rewritten to use `IPythonBuffer`, but still seems like a valid use case.