v0.7.0

This release comes with a new build system based on cargo-xtask.
cargo xtask build and cargo xtask run come with their own --help text.

Also, there are some improvements in the handling of paths:
Paths can now be

• relative to the configuration file
• absolute inside the partition towboot was loaded from (starting with \)
• absolute (starting with fs?:\)

Paths relative to the UEFI shell's current working directory are not supported yet.

Edit: The x86_64 release build is broken for some reason, so here's a debug build.