Get TARGET from the environment

See merge request !11

This enables flexible cross-installation of packages as opposed to only
compiling for the x86_64-unknown-redox target.

This also permits native builds.

Use shadowfile for user creation

See merge request !7

I sorta OOP'ed this, combining the logic of what needs to happen for
each config object with those objects. More things are coming, this is a
very WIP commit.

Maybe some of the refactoring things I did don't make much sense, but I
did them. The goal was to get rid of those macros (because macro
definitions are really hard to read). I replaced two of them with very
similar functions, and rearranged the `install` function.

The installer now generates password hashes from the password entered by
the user or listed in the toml file. This value is NO LONGER the
password hash. The example configs have been changed to reflect this.

Migrate to redox_users

Support adding symlinks in configuration

I'm not sure if this is the best syntax...