Don't generate `-> ()` in `instance_ext`

Be sure to special case `Unit` return types to avoid generating these
unnecessary tokens!

Closes #41

src/gen/instance_ext.rs

 use proc_macro2::TokenStream;
+use quote::ToTokens;
 
-use hir::{Method, Slot, VirtualMethod};
+use hir::{Method, Slot, Ty, VirtualMethod};
 
 use super::signals;
 use hir::names::Names;
@@ -32,9 +33,9 @@ pub fn slot_trait_fns<'ast>(slots: &[Slot<'ast>]) -> Vec<TokenStream> {
             | Slot::VirtualMethod(VirtualMethod { ref sig, .. }) => {
                 let name = &sig.name;
                 let inputs = &sig.inputs;
-                let output = &sig.output;
+                let output = ToReturnType(&sig.output);
                 Some(quote! {
-                    fn #name(#(#inputs),*) -> #output;
+                    fn #name(#(#inputs),*) #output;
                 })
             }
 
@@ -42,9 +43,9 @@ pub fn slot_trait_fns<'ast>(slots: &[Slot<'ast>]) -> Vec<TokenStream> {
                 let connect_signalname = signals::connect_signalname(signal);
                 let sig = &signal.sig;
                 let input_types = signal.sig.input_arg_types();
-                let output = &sig.output;
+                let output = ToReturnType(&sig.output);
                 Some(quote! {
-                    fn #connect_signalname<F: Fn(&Self, #input_types) -> #output + 'static>(&self, f: F) ->
+                    fn #connect_signalname<F: Fn(&Self, #input_types) #output + 'static>(&self, f: F) ->
                         glib::SignalHandlerId;
                 })
             }
@@ -77,9 +78,9 @@ pub fn slot_trait_impls<'ast>(names: &Names, slots: &[Slot<'ast>]) -> Vec<TokenS
                 };
                 let output_from = sig.ret_from_glib_fn(&value);
                 let inputs = &sig.inputs;
-                let output = &sig.output;
+                let output = ToReturnType(&sig.output);
                 Some(quote! {
-                    fn #name(#(#inputs),*) -> #output {
+                    fn #name(#(#inputs),*) #output {
                         #output_from
                     }
                 })
@@ -91,14 +92,14 @@ pub fn slot_trait_impls<'ast>(names: &Names, slots: &[Slot<'ast>]) -> Vec<TokenS
                 let sig = &signal.sig;
                 let signalname_str = format!("{}", sig.name);
                 let input_types = signal.sig.input_arg_types();
-                let output = &sig.output;
+                let output = ToReturnType(&sig.output);
 
                 Some(quote! {
-                    fn #connect_signalname<F: Fn(&Self, #input_types) -> #output + 'static>(&self, f: F) ->
+                    fn #connect_signalname<F: Fn(&Self, #input_types) #output + 'static>(&self, f: F) ->
                         glib::SignalHandlerId
                     {
                         unsafe {
-                            let f: Box<Box<Fn(&Self, #input_types) -> #output + 'static>> =
+                            let f: Box<Box<Fn(&Self, #input_types) #output + 'static>> =
                                 Box::new(Box::new(f));
 
                             glib::signal::connect(self.to_glib_none().0,
@@ -112,3 +113,14 @@ pub fn slot_trait_impls<'ast>(names: &Names, slots: &[Slot<'ast>]) -> Vec<TokenS
         })
         .collect()
 }
+
+struct ToReturnType<'a>(&'a Ty<'a>);
+
+impl<'a> ToTokens for ToReturnType<'a> {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        match self.0 {
+            Ty::Unit => {}
+            other => (quote! { -> #other }).to_tokens(tokens),
+        }
+    }
+}