Remove heim dependency

[fenhl]

The heim crate is currently used for the pid_exists function, but its idiosyncratic dependency specifications make it a compatibility concern. Investigate other ways of checking whether a pid exists so the dependency can be removed:

• The code licence seems to be compatible, so copying the relevant code into this crate is an option. The crate would no longer benefit from updates to heim, but heim seems to be unmaintained anyway.
• Publishing a fork of heim with fixed dependency specifications on crates.io would mean having to maintain an additional crate, but would also benefit other people who have run into the upstream issue.