[Rust] Look through typedefs to pass borrowed strings as &str

[sunfishcode]

wit-bindgen already knows to special-case passing strings by reference and pass them as &str instead of naively passing them as &String and requiring callers to have owned Strings. Implement this behavior for type aliases of strings as well, so that in code like this:

type my-string = string;
foo: func(x: my-string);

the argument is &str instead of &MyString which is effectively &String. And similar for lists.

This comes up in several functions in wasi-http; for example, in the bindings for the Fields::append function, it enables this change:

@@ -5075,8 +5075,8 @@ pub mod wasi {
                 /// `field-value` are syntactically invalid.
                 pub fn append(
                     &self,
-                    name: &FieldName,
-                    value: &FieldValue,
+                    name: &str,
+                    value: &[u8],
                 ) -> Result<(), HeaderError> {
                     unsafe {
                         #[repr(align(1))]

where FieldName and FieldValue are defined as:

            pub type FieldKey = _rt::String;
            pub type FieldName = FieldKey;
            pub type FieldValue = _rt::Vec<u8>;