Support options for reducing page faults

[FlorianUekermann]

Having some options to reduce page faults would be great. On linux the MAP_HUGETLB and MAP_POPULATE options would be the most interesting imo.

[RazrFalcon]

Patches are welcome.

[adamreichold]

@RazrFalcon Concerning patches and the cross-platform target of this crate, would it be alright in your opinion to add an option like readahead to MmapOptions that maps to MAP_POPULATE on Linux but is a no-op on Windows? Or would you prefer a more complex implementation based on posix_madvise for better portability on non-Linux Unix platforms?

(Large pages seem to be available on both platforms even though the Windows variants seems to required more privileges.)

[RazrFalcon]

Yes, I think we could use no-op on Windows. MmapOptions::stack() already does this. Just note this in the docs.