Changed best_matching_locale signature

[skibon02]

PR Type

• Chore (refactoring, formatting, cleanup, etc.)
• Performance
• Usability

Describe changes

1. Changed the best_matching_locale function signature from

   pub fn best_matching_locale<T1, T2>(available_locales: impl Iterator<Item = T1>, user_locales: impl Iterator<Item = T2>) -> Option<String>
   where
   	T1: AsRef<str>,
   	T2: AsRef<str>

   for bcp47, and

   pub fn best_matching_locale<'a, 'b, T1, T2>(available_locales: impl Iterator<Item = &'a T1>, user_locales: impl Iterator<Item = &'b T2>) -> Option<&'a str>
   where
   	T1: AsRef<str> + 'a,
   	T2: AsRef<str> + 'b

   for posix, to

   pub fn best_matching_locale<T1, T2>(available_locales: impl IntoIterator<Item = T1>, user_locales: impl IntoIterator<Item = T2>) -> Option<T1>
   where
   	T1: AsRef<str>,
   	T2: AsRef<str>

   for both.
2. Added tests for template parameter types.
3. Changed the PosixLocale signature from struct PosixLocale<'a> to struct PosixLocale<T: AsRef<str>>.
4. Refactored tests.
5. Updated the code examples in the doc comments and README.md.