Add a raw_module attribute to #[wasm_bindgen]

This allows subverting the checks and resolution performed by the
`module` attribute added as part of [RFC 6] and has been discussed in #1343.

Closes #1343

[RFC 6]: https://github.com/rustwasm/rfcs/pull/6

guide/src/SUMMARY.md

@@ -66,6 +66,7 @@
       - [`js_namespace`](./reference/attributes/on-js-imports/js_namespace.md)
       - [`method`](./reference/attributes/on-js-imports/method.md)
       - [`module = "blah"`](./reference/attributes/on-js-imports/module.md)
+      - [`raw_module = "blah"`](./reference/attributes/on-js-imports/raw_module.md)
       - [`static_method_of = Blah`](./reference/attributes/on-js-imports/static_method_of.md)
       - [`structural`](./reference/attributes/on-js-imports/structural.md)
       - [`variadic`](./reference/attributes/on-js-imports/variadic.md)

guide/src/reference/attributes/on-js-imports/module.md

@@ -31,3 +31,8 @@ generates JavaScript import glue like:
 ```js
 let illmatic = this.illmatic;
 ```
+
+Note that if the string specified with `module` starts with `./`, `../`, or `/`
+then it's interpreted as a path to a [local JS snippet](../../js-snippets.html).
+If this doesn't work for your use case you might be interested in the
+[`raw_module` attribute](raw_module.html)

guide/src/reference/attributes/on-js-imports/raw_module.md

@@ -0,0 +1,19 @@
+# `raw_module = "blah"`
+
+This attribute performs exactly the same purpose as the [`module`
+attribute](module.html) on JS imports, but it does not attempt to interpret
+paths starting with `./`, `../`, or `/` as JS snippets. For example:
+
+```rust
+#[wasm_bindgen(raw_module = "./some/js/file.js")]
+extern "C" {
+    fn the_function();
+}
+```
+
+Note that if you use this attribute with a relative or absolute path, it's
+likely up to the final bundler or project to assign meaning to that path. This
+typically means that the JS file or module will be resolved relative to the
+final location of the wasm file itself. That means that `raw_module` is likely
+unsuitable for libraries on crates.io, but may be usable within end-user
+applications.