Impl TryFrom, From, and Into for VirtAddr #268

Hello,

I was wondering if it would be useful to Implement TryFrom, From, and Into for VirtAddr (and maybe other some types as well). I noticed that VirtAddr type specifically has a to_u64. Reading the docs though it seems like the Into trait might be a more idiomatic way to do this.

Another one that might make sense is the TryFrom for u64 and From trait for u32, u16, u8 (the latter would automatically create an Into according to the docs).

If this seems like a worthwhile feature I'd be happy to make pull request.

Fantastic crate btw!