Fix some situations with duplicate imports (#589)

* Fix importing the same identifier from two modules

This needed a fix in two locations:

* First the generated descriptor function needed its hash to include the module
  that the import came from in order to generate unique descriptor functions.
* Second the generation of the JS shim needed to handle duplicate identifiers in
  a more uniform fashion, ensuring that imported names didn't clash.

* Fix importing the same name in two modules

Previously two descriptor functions with duplicate symbols were emitted, and now
only one function is emitted by using a global table to keep track of state
across macro invocations.

tests/all/duplicates.rs

@@ -0,0 +1,78 @@
+use super::project;
+
+#[test]
+fn same_function_different_locations() {
+    project()
+        .file(
+            "src/lib.rs",
+            r#"
+                #![feature(use_extern_macros)]
+
+                extern crate wasm_bindgen;
+                use wasm_bindgen::prelude::*;
+
+                pub mod a {
+                    use wasm_bindgen::prelude::*;
+                    #[wasm_bindgen(module = "./foo")]
+                    extern {
+                        pub fn foo();
+                    }
+                }
+
+                pub mod b {
+                    use wasm_bindgen::prelude::*;
+                    #[wasm_bindgen(module = "./foo")]
+                    extern {
+                        pub fn foo();
+                    }
+                }
+
+                #[wasm_bindgen]
+                pub fn test() {
+                    a::foo();
+                    b::foo();
+                }
+            "#,
+        )
+        .file("foo.js", "export function foo() {}")
+        .test();
+}
+
+#[test]
+fn same_function_different_modules() {
+    project()
+        .file(
+            "src/lib.rs",
+            r#"
+                #![feature(use_extern_macros)]
+
+                extern crate wasm_bindgen;
+                use wasm_bindgen::prelude::*;
+
+                pub mod a {
+                    use wasm_bindgen::prelude::*;
+                    #[wasm_bindgen(module = "./foo")]
+                    extern {
+                        pub fn foo() -> bool;
+                    }
+                }
+
+                pub mod b {
+                    use wasm_bindgen::prelude::*;
+                    #[wasm_bindgen(module = "./bar")]
+                    extern {
+                        pub fn foo() -> bool;
+                    }
+                }
+
+                #[wasm_bindgen]
+                pub fn test() {
+                    assert!(a::foo());
+                    assert!(!b::foo());
+                }
+            "#,
+        )
+        .file("foo.js", "export function foo() { return true; } ")
+        .file("bar.js", "export function foo() { return false; } ")
+        .test();
+}

tests/all/main.rs

@@ -8,6 +8,7 @@ mod classes;
 mod closures;
 mod comments;
 mod dependencies;
+mod duplicates;
 mod enums;
 mod import_class;
 mod imports;