IndexedDB backend replacement for std::fs

[iceiix]

Local Storage is fine for small files, but for larger binary data it would be more appropriate to use IndexedDB API. Probably as a separate crate.

Emscripten's IndexedDB filesystem (IDBFS) for reference:

https://github.com/emscripten-core/emscripten/blob/incoming/src/IDBStore.js
https://github.com/emscripten-core/emscripten/blob/incoming/src/library_idbstore.js
https://github.com/emscripten-core/emscripten/blob/incoming/src/library_idbfs.js

Mozilla documentation: https://developer.mozilla.org/en-US/docs/Web/API/IndexedDB_API https://developer.mozilla.org/en-US/docs/Web/API/IndexedDB_API/Using_IndexedDB

Rust wasm support for IDB? Could be a good idea to use stdweb, updated koute/stdweb#342 from koute/stdweb#297 awaiting review. Or adapt Emscripten's .js files?