feat!: gix-config convenience initiative

A new `gix_config::AsKey` trait allows all `gix_config::File` methods
to receive an implementation of `AsKey` instead of the tuple of
`(section, subsection, value_name)`. This is the default, and all
methods that take a tuple have been renamed to `<name>_by()`, note the `_by` suffix.

The terminology was reworked so that `key` is now only used to identify a value.

This change also affects the public API of `gix`, which provides
ways to set values on configuration snapshots which now by default
will only take a `AsKey` implementation.

Note that `gix::config::tree::*` keys now also implement `AsKey`,
which allows them to be used more conveniently when setting values,
too.

Author: Byron

gix-config/fuzz/fuzz_targets/fuzz_file.rs

@@ -17,7 +17,7 @@ fn fuzz_immutable_section(section: &gix_config::file::Section<'_>, buf: &mut Vec
         let _ = black_box((key, value));
     }
     let mut seen = BTreeSet::new();
-    for key in section.keys() {
+    for key in section.value_names() {
         if seen.insert(key) {
             let _ = black_box(section.values(key.as_ref()));
         }
@@ -37,7 +37,7 @@ fn fuzz_mutable_section(
     // Mutate section.
     let section_id = {
         let mut section = file.section_mut(section_name, subsection_name)?;
-        let key = section.keys().next().cloned();
+        let key = section.value_names().next().cloned();
 
         if let Some(key) = key {
             section.push_newline();

gix-config/src/file/access/comfort.rs

@@ -172,7 +172,7 @@ impl<'event> File<'event> {
     /// # use gix_config::parse::section;
     /// let mut git_config = gix_config::File::default();
     /// let mut section = git_config.new_section("hello", Some(Cow::Borrowed("world".into())))?;
-    /// section.push(section::Key::try_from("a")?, Some("b".into()));
+    /// section.push(section::ValueName::try_from("a")?, Some("b".into()));
     /// let nl = section.newline().to_owned();
     /// assert_eq!(git_config.to_string(), format!("[hello \"world\"]{nl}\ta = b{nl}"));
     /// let _section = git_config.new_section("core", None);