Fix some hints from Rust and clippy

* the feature `proc_macro` has been stable since 1.29.0 and no longer
  requires an attribute to enable

* https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#needless_return

* https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#len_zero

src/api/request.rs

@@ -181,7 +181,7 @@ impl ToTokens for Request {
             pub struct Request
         };
 
-        let request_struct_body = if self.fields.len() == 0 {
+        let request_struct_body = if self.fields.is_empty() {
             quote!(;)
         } else {
             let fields = self.fields.iter().fold(TokenStream::new(), |mut field_tokens, request_field| {

src/api/response.rs

@@ -15,7 +15,7 @@ impl Response {
     }
 
     pub fn has_fields(&self) -> bool {
-        self.fields.len() != 0
+        !self.fields.is_empty()
     }
 
     pub fn has_header_fields(&self) -> bool {
@@ -142,7 +142,7 @@ impl From<Vec<Field>> for Response {
                     if has_newtype_body {
                         panic!("ruma_api! responses cannot have both normal body fields and a newtype body field");
                     } else {
-                        return ResponseField::Body(field);
+                        ResponseField::Body(field)
                     }
                 }
                 ResponseFieldKind::Header => ResponseField::Header(field, header.expect("missing header name")),
@@ -164,7 +164,7 @@ impl ToTokens for Response {
             pub struct Response
         };
 
-        let response_struct_body = if self.fields.len() == 0 {
+        let response_struct_body = if self.fields.is_empty() {
             quote!(;)
         } else {
             let fields = self.fields.iter().fold(TokenStream::new(), |mut fields_tokens, response_field| {

src/lib.rs

@@ -4,7 +4,6 @@
 //! See the documentation for the `ruma_api!` macro for usage details.
 
 #![deny(missing_debug_implementations)]
-#![feature(proc_macro)]
 #![recursion_limit="256"]
 
 extern crate proc_macro;

tests/ruma_api_macros.rs

@@ -1,4 +1,4 @@
-#![feature(proc_macro, try_from)]
+#![feature(try_from)]
 
 extern crate futures;
 extern crate http;