Don't generate duplicate thunks

Previously, functions that appear in multiple places
(exported, start function, table) would generate a thunk
for each place they appear in. Those additional thunks are
identical and only only one of them would be referenced.
Main offender are tables with redundant entries.

This commit eliminates those duplicate thunks without adding
any additional overhead.

src/stack_height/thunk.rs

@@ -13,7 +13,6 @@ struct Thunk {
 	signature: FunctionType,
 	// Index in function space of this thunk.
 	idx: Option<u32>,
-	original_func_idx: u32,
 	callee_stack_cost: u32,
 }
 
@@ -23,9 +22,6 @@ pub(crate) fn generate_thunks(
 ) -> Result<elements::Module, Error> {
 	// First, we need to collect all function indices that should be replaced by thunks
 
-	// Function indices which needs to generate thunks.
-	let mut need_thunks: Vec<u32> = Vec::new();
-
 	let mut replacement_map: Map<u32, Thunk> = {
 		let exports = module
 			.export_section()
@@ -57,12 +53,10 @@ pub(crate) fn generate_thunks(
 
 			// Don't generate a thunk if stack_cost of a callee is zero.
 			if callee_stack_cost != 0 {
-				need_thunks.push(func_idx);
 				replacement_map.insert(func_idx, Thunk {
 					signature: resolve_func_type(func_idx, &module)?.clone(),
 					idx: None,
 					callee_stack_cost,
-					original_func_idx: func_idx,
 				});
 			}
 		}
@@ -76,17 +70,9 @@ pub(crate) fn generate_thunks(
 	let mut next_func_idx = module.functions_space() as u32;
 
 	let mut mbuilder = builder::from_module(module);
-	for func_idx in need_thunks {
-		let mut thunk = replacement_map
-			.get_mut(&func_idx)
-			.expect(
-				"`func_idx` should come from `need_thunks`;
-				`need_thunks` is populated with the same items that in `replacement_map`;
-				qed"
-			);
-
+	for (func_idx, thunk) in replacement_map.iter_mut() {
 		let instrumented_call = instrument_call!(
-			thunk.original_func_idx as u32,
+			*func_idx,
 			thunk.callee_stack_cost as i32,
 			ctx.stack_height_global_idx(),
 			ctx.stack_limit()
@@ -113,7 +99,7 @@ pub(crate) fn generate_thunks(
 				// Signature of the thunk should match the original function signature.
 				.signature()
 					.with_params(thunk.signature.params().to_vec())
-					.with_return_type(thunk.signature.return_type().clone())
+					.with_return_type(thunk.signature.return_type())
 					.build()
 				.body()
 					.with_instructions(elements::Instructions::new(