Refactor creation of functions in the backend

This commit refactors the lowest-level primitive for creating functions into a
new `create_one_function` function. This doesn't take into account overloading
but is suitable for things like `create_{getter,setter}`. Eventually the
overloading will be implemented in terms of this function.

crates/webidl/src/lib.rs

@@ -577,7 +577,7 @@ impl<'src> FirstPassRecord<'src> {
         use weedle::interface::Special;
 
         let is_static = match modifier {
-            Some(Stringifier(_)) => uniimplemented!(), // filtered out earlier
+            Some(Stringifier(_)) => unimplemented!(), // filtered out earlier
             Some(Static(_)) => true,
             None => false,
         };