RFE: allow mapping from a win32 mapping object

[elmarco]

In my use case, the process is handed over a duplicated handle of the mapping object. I would need memmap2 to allow mapping from it directly (since I don't see a way to make a file handle from the mapping object).

One way would be to make an enum of MmapRawDescriptor on win32, and add a constructor for mapping object.

Any other idea?
Thanks