Add a #[wasm_bindgen(start)] attribute

This commit adds a new attribute to `#[wasm_bindgen]`: `start`. The
`start` attribute can be used to indicate that a function should be
executed when the module is loaded, configuring the `start` function of
the wasm executable. While this doesn't necessarily literally configure
the `start` section, it does its best!

Only one crate in a crate graph may indicate `#[wasm_bindgen(start)]`,
so it's not recommended to be used in libraries but only end-user
applications. Currently this still must be used with the `crate-type =
["cdylib"]` annotation in `Cargo.toml`.

The implementation here is somewhat tricky because of the circular
dependency between our generated JS and the wasm file that we emit. This
circular dependency makes running initialization routines (like the
`start` shim) particularly fraught with complications because one may
need to run before the other but bundlers may not necessarily respect
it. Workarounds have been implemented for various emission strategies,
for example calling the start function directly after exports are wired
up with `--no-modules` and otherwise working around what appears to be
a Webpack bug with initializers running in a different order than we'd
like. In any case, this in theory doesn't show up to the end user!

Closes #74

guide/src/SUMMARY.md

@@ -76,6 +76,7 @@
       - [`constructor`](./reference/attributes/on-rust-exports/constructor.md)
       - [`js_name = Blah`](./reference/attributes/on-rust-exports/js_name.md)
       - [`readonly`](./reference/attributes/on-rust-exports/readonly.md)
+      - [`start`](./reference/attributes/on-rust-exports/start.md)
       - [`typescript_custom_section`](./reference/attributes/on-rust-exports/typescript_custom_section.md)
 
 --------------------------------------------------------------------------------

guide/src/reference/attributes/on-rust-exports/start.md

@@ -0,0 +1,31 @@
+# `start`
+
+When attached to a `pub` function this attribute will configure the `start`
+section of the wasm executable to be emitted, executing the tagged function as
+soon as the wasm module is instantiated.
+
+```rust
+#[wasm_bindgen(start)]
+pub fn main() {
+    // executed automatically ...
+}
+```
+
+The `start` section of the wasm executable will be configured to execute the
+`main` function here as soon as it can. Note that due to various practical
+limitations today the start section of the executable may not literally point to
+`main`, but the `main` function here should be started up automatically when the
+wasm module is loaded.
+
+There's a few caveats to be aware of when using the `start` attribute:
+
+* The `start` function must take no arguments and must either return `()` or
+  `Result<(), JsValue>`
+* Only one `start` function can be placed into a module, including its
+  dependencies. If more than one is specified then `wasm-bindgen` will fail when
+  the CLI is run. It's recommended that only applications use this attribute.
+* The `start` function will not be executed when testing.
+* If you're experimenting with WebAssembly threads, the `start` function is
+  executed *once per thread*, not once globally!
+* Note that the `start` function is relatively new, so if you find any bugs with
+  it, please feel free to report an issue!