Add madvise wrapper

[eira-fransham]

This doesn't have docs for AccessPattern yet and hasn't been properly tested, but it's a start. Worth getting some critique early. It also explicitly only implements the POSIX madvise flags instead of including the Linux extensions, although that would definitely be a good thing to add.

[danburkert]

Hi @Vurich, I've previously said it would be interesting to have a madvise wrapper in memmap, but I'm going to play the opposite side for a moment and argue against it. I think this functionality might be better exposed in a generic way outside this crate, because it's not really specific to memory maps, and the implementation is not tied to the memory map internals at all. Is there a compelling reason to tie it to memory maps that I'm overlooking?

[eira-fransham]

No, I think you're right. It could be implemented as a method on [u8] with very little syntactic overhead.

[eira-fransham]

Made this into a seperate crate here.