Add IndexedDB and web-sys example to examples/ and guide

[fitzgen]

General info on IndexedDB: https://developer.mozilla.org/en-US/docs/Web/API/IndexedDB_API

• Add an example to examples/ that shows off working with IndexedDB
• Add a subsection to "web-sys >> examples" in the guide that includes this example

[alexcrichton]

I"m gonna close this in favor of rustwasm/gloo#68 because I think IndexedDB is more likely to be covered by gloo rather than wasm-bindgen examples

[indolering]

@alexandrestein rustwasm/gloo#68 has been closed and won't be implemented.

[Pauan]

@indolering We still plan to add an API for IndexedDB, though probably not that specific API.

[datdinhquoc]

Please, examples for IndexedDB in web-sys, I can't get it working.

Got the IdbDatabase, got IdbObjectStore, but failed to add a doc:

// The store is already in db with key id, autoIncremental
myStore.add(&JsValue::from_str("{test:1}")).unwrap();

Failed at '.unwrap()', although it's a Result.

Error in browser console:

Uncaught RuntimeError: unreachable
...
at _dyn_core__ops__function__FnMut__A____Output___R_as_wasm_bindgen__closure__WasmClosure___describe__invoke__h12d73e1d41de1f27
at __wbg_adapter_20