Fix some build errors

I fixed (hopefully) some build errors popping up with a recent nightly.

I'm quite new to rust, so please double check.

This pull request is incomplete, I'm still unable to fix the (last?) error:
`../rust-argparse/src/parser.rs:515:13: 515:43 error: cannot infer an appropriate lifetime for autoref due to conflicting requirements`
in fact 3 similar ones. But I'm working on that too :)