modernization: tweaks for c++20

Update various places for what we can do with C++20:

- ends_with / starts_with
- std::to_array
- using instead of typedef
- designated initializers
- "[[maybe_unused]]" instead of G_GNUC_UNUSED

Also remove some unnecessary 'static'

lib/utils/mu-result.hh

@@ -48,7 +48,7 @@ Ok(T&& t)
  *
  * @return a success Result<void>
  */
-static inline Result<void>
+inline Result<void>
 Ok()
 {
 	return {};
@@ -72,27 +72,27 @@ Err(const Error& err)
 	return tl::unexpected(err);
 }
 
-static inline tl::unexpected<Error>
+inline tl::unexpected<Error>
 Err(Error&& err)
 {
 	return tl::unexpected(std::move(err));
 }
 
-static inline tl::unexpected<Error>
+inline tl::unexpected<Error>
 Err(const Error& err)
 {
 	return tl::unexpected(err);
 }
 
 template<typename T>
-static inline tl::unexpected<Error>
+inline tl::unexpected<Error>
 Err(const Result<T>& res)
 {
 	return res.error();
 }
 
 template<typename T>
-static inline tl::unexpected<Error>
+inline tl::unexpected<Error>
 Err(Result<T>&& res)
 {
 	return std::move(res.error());