Add MAP_FIXED mode

[danburkert]

Add a construction option for creating MAP_FIXED memory maps at a known memory location. CC #20 .

[darfink]

I was thinking about implementing this but saw that CreateFileMappingW is used on Windows for anonymous maps. Is there any specific reason VirtualAlloc is not used instead?

[darfink]

Is it possible for this to be implemented in before the crate evaluation? Right now this is the only missing feature, preventing me from publishing my detour library.

I've been constrained to retep998's unpublished fork of rust-mmap.

As previously mentioned I could go on and try to implement this myself but I believe there are some underlying reasons as to why CreateFileMapping is used on Windows (IPC?).

[dtolnay]

Is this something that can be added backward compatibly after 1.0?

[danburkert]

Yes, as I noted in #48 (comment) I think we can do this backwards compatibly post 1.0.