Reimplement DSL

[squell]

Closes #933

This also isolates all the 'Defaults'-behaviour in the defaults crate, making the exact behaviour of each individual Defaults setting irrelevant to the way the parser and semantic analysis handles them. This simplifies those parts.

The DSL definition itself also feels a little less magical (although there's some syntax-rules hacks there, it's slightly more "regular").

E.g. it now generates:

   struct Settings {
        always_query_group_plugin: bool,
        always_set_home: bool,
        env_reset: bool,
        mail_badpass: bool,
        match_group_by_gid: bool,
        use_pty: bool,
        visiblepw: bool,
        env_editor: bool,
        passwd_tries: i64,
        secure_path: Option<Box<str>>,
        verifypw: crate::defaults::enums::verifypw,
        timestamp_timeout: i64,
        env_keep: std::collections::HashSet<String>,
        env_check: std::collections::HashSet<String>,
        env_delete: std::collections::HashSet<String>,
    }

And impls Default for this struct, as well as define a fn negate and fn set that construct an object of the type pub type SettingsModifier = Box<dyn FnOnce(&mut Settings)>;. E.g.,

            "use_pty" => {
                Some(
                    crate::defaults::SettingKind::Flag(
                        Box::new(move |obj: &mut Settings| obj.use_pty = true),
                    ),
                )
            }

Note also that the enum-like sudoers settings are turned into an actual enum (albeit necessarily with uncommon name casing, but it's tucked away in defaults::enums)

    #[allow(non_camel_case_types)]
    mod enums {
        pub enum verifypw {
            all,
            always,
            any,
            never,
        }

Also, the DSL is built so you get compiler warnings if you define variabels that you do not use:

warning: multiple methods are never used
   --> src/defaults/mod.rs:24:5
    |
24  |     always_query_group_plugin = false
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^
25  |     always_set_home           = false
    |     ^^^^^^^^^^^^^^^
26  |     env_reset                 = true
    |     ^^^^^^^^^
27  |     mail_badpass              = true
    |     ^^^^^^^^^^^^
28  |     match_group_by_gid        = false
    |     ^^^^^^^^^^^^^^^^^^
29  |     use_pty                   = true
30  |     visiblepw                 = false
    |     ^^^^^^^^^
...
37  |     verifypw                  = all (!= never) [all, always, any, never]
    |     ^^^^^^^^
...
46  |     env_delete                = ["IFS", "CDPATH", "LOCALDOMAIN", "RES_OPTIONS", "HOSTALIASES",
    |     ^^^^^^^^^^
    |
   ::: src/defaults/settings_dsl.rs:172:9
    |
172 |         impl Settings {
    |         ------------- methods in this implementation
    |
    = note: `#[warn(dead_code)]` on by default
    = note: this warning originates in the macro `defaults` (in Nightly builds, run with -Z macro-backtrace for more info)

warning: `sudo-rs` (lib) generated 1 warning
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.61s