Add is_eof() function to Parse trait

[xq-tec]

Use Parse::is_eof() to replace the previous check for end-of-input.

Fixes #251

[xq-tec]

Whoops, I accidentally used the wrong login for the pull request.
Anyway, thank you for your awesome work on rust-peg!

[kevinmehall]

Thanks!

Be aware that it might not make it into a release for a little while, as it's a breaking change for anyone who has their own Parse implementation, and there are a few other breaking changes I want to get in to 0.7.