Turn everything into OsString and Just Walk Out! for strings with nul. To reduce the hazard surface of the API, this replaces most uses of &str with &OsStr (and likewise with String/OsString). Also, I've decided that instead of dealing with callers putting `\0` in their parameters, I'm going to follow the example of std::env and Just Walk Out! (i.e., panic!()). This makes things a lot less annoying for both me and (hopefully) users.

FROM docker.io/debian:stable

RUN apt-get -y update \
    && apt-get -y upgrade \
    && apt-get install -y cargo mercurial \
    && apt-get clean