Expose page_size pub function as part of crate api

[scriptandcompile]

Currently I'm creating my pages using a non cross platform independent API set. I'm basically using a modified version of this crate where page_size is exposed for windows and linux. I'm unsure of how to do this same thing on some of the other platforms memmap-rs supports or if it's the same as the linux call.

Would you be interested in a pull request with these calls public and cross platform implemented?