Expand the proc-macro composability test vector (#1600)

### What
Add parse_item_impl attribute macro that parses impl blocks and apply it
to contractimpl to test macro composition.

  ### Why
The existing tests only validated function-level macro composition. This
verifies that soroban-sdk macros compose correctly with impl-level
attribute macros. What's under test is pretty minor but easy to test to
include so why not.

For #1544

Author: leighmcculloch

tests-expanded/test_macros_tests.rs

@@ -4,7 +4,7 @@
 extern crate core;
 #[prelude_import]
 use core::prelude::rust_2021::*;
-use proc_macros::parse_item_fn;
+use proc_macros::{parse_item_fn, parse_item_impl};
 use soroban_sdk::{contract, contractimpl};
 pub struct Contract;
 ///ContractArgs is a type for building arg lists for functions defined in "Contract".
@@ -313,9 +313,9 @@ mod test {
             ignore: false,
             ignore_message: ::core::option::Option::None,
             source_file: "tests/macros/src/lib.rs",
-            start_line: 25usize,
+            start_line: 26usize,
             start_col: 8usize,
-            end_line: 25usize,
+            end_line: 26usize,
             end_col: 18usize,
             compile_fail: false,
             no_run: false,

tests-expanded/test_macros_wasm32v1-none.rs

@@ -4,7 +4,7 @@
 extern crate core;
 #[prelude_import]
 use core::prelude::rust_2021::*;
-use proc_macros::parse_item_fn;
+use proc_macros::{parse_item_fn, parse_item_impl};
 use soroban_sdk::{contract, contractimpl};
 pub struct Contract;
 ///ContractArgs is a type for building arg lists for functions defined in "Contract".

tests/macros/proc_macros/src/lib.rs

@@ -2,7 +2,14 @@
 /// soroban-sdk macros are composable and compatible with a variety of other macros.
 use proc_macro::TokenStream;
 use quote::quote;
-use syn::{parse_macro_input, ItemFn};
+use syn::{parse_macro_input, ItemFn, ItemImpl};
+
+/// An attribute macro that expects to be used on an impl.
+#[proc_macro_attribute]
+pub fn parse_item_impl(_metadata: TokenStream, input: TokenStream) -> TokenStream {
+    let item = parse_macro_input!(input as ItemImpl);
+    quote! { #item }.into()
+}
 
 /// An attribute macro that expects to be used on a function.
 #[proc_macro_attribute]

tests/macros/src/lib.rs

@@ -2,13 +2,14 @@
 // validating that they are composable and compatible.
 
 #![no_std]
-use proc_macros::parse_item_fn;
+use proc_macros::{parse_item_fn, parse_item_impl};
 use soroban_sdk::{contract, contractimpl};
 
 #[contract]
 pub struct Contract;
 
 #[contractimpl]
+#[parse_item_impl]
 impl Contract {
     // Test that attribute macros that expect to be used on fns are composable with contractimpl.
     #[parse_item_fn]