Add FuncEnvironment hooks to generate prologue and epilogue code.

In some cases, it is useful to do some work at entry to or exit from a
Cranelift function translated from WebAssembly. This PR adds two
optional methods to the `FuncEnvironment` trait to do just this,
analogous to the pre/post-hooks on operators that already exist.

This PR also includes a drive-by compilation fix due to the latest
nightly wherein `.is_empty()` on a `Range` ambiguously refers to either
the `Range` impl or the `ExactSizeIterator` impl and can't resolve.

cranelift/wasm/src/environ/spec.rs

@@ -624,6 +624,26 @@ pub trait FuncEnvironment: TargetEnvironment {
     ) -> WasmResult<()> {
         Ok(())
     }
+
+    /// Optional callback for the `FunctionEnvironment` performing this translation to perform work
+    /// before the function body is translated.
+    fn before_translate_function(
+        &mut self,
+        _builder: &mut FunctionBuilder,
+        _state: &FuncTranslationState,
+    ) -> WasmResult<()> {
+        Ok(())
+    }
+
+    /// Optional callback for the `FunctionEnvironment` performing this translation to perform work
+    /// after the function body is translated.
+    fn after_translate_function(
+        &mut self,
+        _builder: &mut FunctionBuilder,
+        _state: &FuncTranslationState,
+    ) -> WasmResult<()> {
+        Ok(())
+    }
 }
 
 /// An object satisfying the `ModuleEnvironment` trait can be passed as argument to the