rename main -> start on wasm-bindgen(start) example

[kazuk]

I got error with this example.

error: entry symbol `main` declared multiple times
  |
  = help: did you use `#[no_mangle]` on `fn main`? Use `#[start]` instead

In this document, "main" is specified as the function name. However, compilation will fail due to the previously mentioned error if "main" is used. This is presumably because "main" is a function name reserved in Rust as the starting function for binaries.

rename to start then succeeded.

[daxpedda]

This is a bit surprising to me, but I believe it's better this way anyway to differentiate #[wasm_bindgen(start)] from #[wasm_bindgen(main)].

There are bunch of mentions to the main() functions in this file, those would need to be updated to start as well.

[kazuk]

I re-test this. I got build error on wasm-pack test.

So main symbol conflicting with rust's cargo test generated main.

[daxpedda]

Thank you!