Implement the local JS snippets RFC

This commit is an implementation of [RFC 6] which enables crates to
inline local JS snippets into the final output artifact of
`wasm-bindgen`. This is accompanied with a few minor breaking changes
which are intended to be relatively minor in practice:

* The `module` attribute disallows paths starting with `./` and `../`.
  It requires paths starting with `/` to actually exist on the filesystem.
* The `--browser` flag no longer emits bundler-compatible code, but
  rather emits an ES module that can be natively loaded into a browser.

Otherwise be sure to check out [the RFC][RFC 6] for more details, and
otherwise this should implement at least the MVP version of the RFC!
Notably at this time JS snippets with `--nodejs` or `--no-modules` are
not supported and will unconditionally generate an error.

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

Closes #1311

tests/headless/snippets.rs

@@ -0,0 +1,42 @@
+use wasm_bindgen::prelude::*;
+use wasm_bindgen_test::*;
+
+#[wasm_bindgen(module = "/tests/headless/snippets1.js")]
+extern {
+    fn get_two() -> u32;
+}
+
+#[wasm_bindgen_test]
+fn test_get_two() {
+    assert_eq!(get_two(), 2);
+}
+
+#[wasm_bindgen(inline_js = "export function get_three() { return 3; }")]
+extern {
+    fn get_three() -> u32;
+}
+
+#[wasm_bindgen_test]
+fn test_get_three() {
+    assert_eq!(get_three(), 3);
+}
+
+#[wasm_bindgen(inline_js = "let a = 0; export function get() { a += 1; return a; }")]
+extern {
+    #[wasm_bindgen(js_name = get)]
+    fn duplicate1() -> u32;
+}
+
+#[wasm_bindgen(inline_js = "let a = 0; export function get() { a += 1; return a; }")]
+extern {
+    #[wasm_bindgen(js_name = get)]
+    fn duplicate2() -> u32;
+}
+
+#[wasm_bindgen_test]
+fn duplicate_inline_not_unified() {
+    assert_eq!(duplicate1(), 1);
+    assert_eq!(duplicate2(), 1);
+    assert_eq!(duplicate1(), 2);
+    assert_eq!(duplicate2(), 2);
+}