[IronPython] Convert between python array and .NET Array

[Dino Viehland]

Marcel wrote:
> Thanks Dino, the ctypes.memmove does work!
> 
> I wasn't aware that ctypes could be used in IronPython. Do they
> marshal to unmanaged code under the hood?

There's not really any unmanaged code on *our* side involved at all.  It
all turns into .NET's support for interop.  So we'll generate calli calls
w/ the correct signature.  And then all the memory access we do is through
the Marshal APIs.  I actually didn't even need to flip the /unsafe compiler
flag in order to implement it all :)  

> 
> Let me know if you want me to file a bug against the Marshal.Copy
> overload behavior.

I actually don't think this is a bug.  The problem here is that you either
have a pointer (as an int but it could trivially be turned into an IntPtr) or 
an IronPython array object and then an IntPtr.  For the 2 IntPtr cases there's
no overloads on Marshal and I don't believe anywhere in .NET.  For ctypes to
implement memmove we had to P/Invoke.  And for the array case you don't have
a .NET array.

If we exposed the underlying .NET array then you could use Marshal.Copy.  But 
I'm reluctant to do that because we might want to switch to allocating 
unmanaged memory for the array in the future.  That'll work better w/ ctypes 
and calls to buffer_info() - currently need to pin the array for 
an unknown amount of time.  That's another reason to allocate the memory via
ctypes.