Update documentation

See merge request !37

I've made the following changes to make pkg more user-friendly:
    - Added an 'about' string to every subcommand (keeping the old help
      messages that were present before clap)
    - Added a basic description to the application
    - Reformatted some of the code due to rustfmt

Use Rust 2018 edition

See merge request redox-os/pkgutils!36

Update Cargo.lock

See merge request !33

AArch64 builds need a newer syscall crate.

update to use liner fork

Dependencies are handled by looking for a package manifest file in the
local package database. The manifest contains all the dependencies for a
package, and a dependency graph is built from all of them. Then a
topological sort is performed and an OrderMap (linked hash map) of
package names in the order which they are to be installed in is
returned. An OrderMap is used because it preserves the order of the
packages and it has fast lookup times for checking duplicate packages.

Use BufWriter