add swap support

[davidkna]

This PR adds swap to Platform and implements the required functionality for Linux, macOS and Windows. Further, TryFrom is implemented between Memory and Swap based on per-platform compatibility (Linux and Windows are supported).

Closes #93

[valpackett]

hmm. I don't think the TryFrom thing is a very good API. The simpler, more obvious way to leverage the fact that some platforms provide both in one syscall would be to just have all the retrieval options as separate functions:

• fn memory(&self) -> io::Result<Memory>
• fn swap(&self) -> io::Result<Swap>
• fn memory_and_swap(&self) -> io::Result<(Memory, Swap)> /// On some platforms this is more efficient than calling memory() and swap() separately

[davidkna]

Done!

[valpackett]

Thanks!