Debloat the binary by using less AsRef arguments

Using `impl AsRef<str>` as the argument instead of `&str` makes it
possible to call the function with `&str`, `String` and other types
that implement `AsRef` trait.

The cost of it is that compiled binary contains mulitple versions of
the same function, one for each variant of types. If function contains
multiple generic `impl AsRef` arguments, the number of versions possibly
compiled into binary grows exponentially with the number of arguments.

Simple way to avoid it is to call `.as_ref()` on the caller side to
convert the argument to `&str`. In most cases even adding a `&` and
relying on `Deref` coercion is sufficient.

This patch changes many functions that accepted `impl AsRef<str>` and
`impl AsRef<Path>` to accept `&str` and `&Path` instead.

In some places `.clone()` calls are removed. Calling `.clone()` on
`String` and passing `String` to a function accepting `impl
AsRef<str>` is completely unnecessary as `&str` reference could be
passed instead. There is no clippy warning against it yet, but
changing argument type to `&str` allowed to find these cases.

The result of debloating is not impressive, several hundred kilobytes
are saved, which is about 3% of the `.so` binary, but the code is
cleaner too.

src/provider.rs

@@ -115,14 +115,14 @@ pub fn get_provider_by_domain(domain: &str) -> Option<&'static Provider> {
 /// Finds a provider based on MX record for the given domain.
 ///
 /// For security reasons, only Gmail can be configured this way.
-pub async fn get_provider_by_mx(domain: impl AsRef<str>) -> Option<&'static Provider> {
+pub async fn get_provider_by_mx(domain: &str) -> Option<&'static Provider> {
     if let Ok(resolver) = resolver(
         config::ResolverConfig::default(),
         config::ResolverOpts::default(),
     )
     .await
     {
-        let mut fqdn: String = String::from(domain.as_ref());
+        let mut fqdn: String = domain.to_string();
         if !fqdn.ends_with('.') {
             fqdn.push('.');
         }