Add a simple object with methods

This currently fails, because it generates this code:

    uniffi::call_with_output(call_status, || {
        Counter::add(
            &<std::sync::Arc as uniffi::ViaFfi>::try_lift(ptr).unwrap(),
            <u32 as uniffi::ViaFfi>::try_lift(amount).unwrap(),
        )
    })

Precisely this line:

    &<std::sync::Arc as uniffi::ViaFfi>::try_lift(ptr).unwrap(),

Which leads to this error (with hilarious fix suggestions):

    9   | #[uniffi::declare_interface]
        | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected 1 type argument
        |
    note: struct defined here, with 1 type parameter: `T`
        = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)
    help: use angle brackets to add missing type argument
        |
    9   | #[uniffi::declare_interface]<T>
        |                             ^^^

The `Arc` is missing its type argument.
That code comes from here: `uniffi_bindgen/src/scaffolding/mod.rs#L95`
I'm not sure yet where we lose the type information.

examples/geometry/src/lib.rs

@@ -16,6 +16,16 @@ mod geometry {
         pub coord_y: f64,
     }
 
+    pub struct Counter {
+        value: u32
+    }
+
+    impl Counter {
+        fn add(&self, amount: u32) {
+            println!("amount: {}", amount);
+        }
+    }
+
     #[derive(Debug, Clone)]
     pub struct Line {
         pub start: Point,