Move return_at_end out of Settings and into the wasm FuncEnvironment. (#547)

* Move `return_at_end` out of Settings and into the wasm FuncEnvironment.

The `return_at_end` flag supports users that want to append a custom
epilogue to Cranelift-produced functions. It arranges for functions to
always return via a single return statement at the end, and users are
expected to remove this return to append their code.

This patch makes two changes:
 - First, introduce a `fallthrough_return` instruction and use that
   instead of adding a `return` at the end. That's simpler than having
   users remove the `return` themselves.

 - Second, move this setting out of the Settings and into the wasm
   FuncEnvironment. This flag isn't something the code generator uses,
   it's something that the wasm translator uses. The code generator
   needs to preserve the property, however we can give the
   `fallthrough_return` instruction properties to ensure this as needed,
   such as marking it non-cloneable.

lib/wasm/src/environ/spec.rs

@@ -74,6 +74,15 @@ impl WasmError {
 /// A convenient alias for a `Result` that uses `WasmError` as the error type.
 pub type WasmResult<T> = Result<T, WasmError>;
 
+/// How to return from functions.
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+pub enum ReturnMode {
+    /// Use normal return instructions as needed.
+    NormalReturns,
+    /// Use a single fallthrough return at the end of the function.
+    FallthroughReturn,
+}
+
 /// Environment affecting the translation of a single WebAssembly function.
 ///
 /// A `FuncEnvironment` trait object is required to translate a WebAssembly function to Cranelift
@@ -216,6 +225,11 @@ pub trait FuncEnvironment {
     fn translate_loop_header(&mut self, _pos: FuncCursor) {
         // By default, don't emit anything.
     }
+
+    /// Should the code be structured to use a single `fallthrough_return` instruction at the end
+    /// of the function body, rather than `return` instructions as needed? This is used by VMs
+    /// to append custom epilogues.
+    fn return_mode(&self) -> ReturnMode;
 }
 
 /// An object satisfying the `ModuleEnvironment` trait can be passed as argument to the