Upgrade to 2018 edition

[tesuji]

This PR is based on decision from #40 of @m4b : #40 (comment)

[m4b]

@lzutao thank you for this! I want to land this but I really wanted to see @willglynn PR here landed first, which was a really great improvement, namely it allows:

1. pwriting extremely large values out of boxes, instead of on the stack (which will usually blow up the stack)
2. better user/api ergonomics by not requiring owned values in pwrite calls.

Let me see what status is, if not maybe I or someone else can try to rebase and fix.

[m4b]

actually this probably has minor conflicts? let me check

[m4b]

There's only one conflict, and it's minor, thanks again for pushing us into 2018 @lzutao :)