Group parsing

[InnocentZero]

Adds TOML parsing. Some backend traits need changes to make them TOML compatible. Else we'd need to change the group file description language.

[InnocentZero]

In particular, unit type () doesn't work. It has been used in a lot of places so we either need to find alternatives for backend types or find a different config file format.

[ripytide]

I think the best option so we don't have to throw away the strongly typed backend associated trait types might be to write a bit of custom serialization/deserialization logic so that something like this gets mapped into the BTreeMaps inside PackagesInstall:

arch = [
"pacdef-bin",
{ package="neovim", clean_install = true },
]

[ripytide]

Semantically this is roughly equivalent to a BTreeSet<struct { package: String, clean_install: Option<bool>,} but then we lose the uniqueness guarantee from the BTreeMap

[ripytide]

I rebased the PR after #3 was merged

[ripytide]

I think we might need to finalize our ideal file format first in #5 and then implementing it will probably be best via a custom parsing function that builds a struct from a generic toml::Table type without serde due to the difficulties with getting serde to serialize/deserialize exactly the way we want.

[ripytide]

The PR looks good, I'm going to merge this so I can take a shot at implementing the file format from #5 in another PR