[wasm] Make FuncEnvironment functions fallible (fixes #752);

cranelift/wasm/src/lib.rs

@@ -38,9 +38,16 @@ extern crate alloc as std;
 extern crate std;
 
 #[cfg(not(feature = "std"))]
-use hashmap_core::{map as hash_map, HashMap};
+use hashmap_core::{
+    hash_map::Entry::{Occupied, Vacant},
+    map as hash_map, HashMap,
+};
 #[cfg(feature = "std")]
-use std::collections::{hash_map, HashMap};
+use std::collections::{
+    hash_map,
+    hash_map::Entry::{Occupied, Vacant},
+    HashMap,
+};
 
 mod code_translator;
 mod environ;