Investigate kv-storage standard proposal

[cyrilletuzi]

There is a standard proposal called kv-storage (for key-value storage), which goal is exactly the same as the main goal of this lib: having a simple key-value storage like localStorage, but asynchronous to be performant via indexedDB.

Investigation is needed to prepare to this new API, to be sure the lib API is consistent.

[cyrilletuzi]

Done in #100

.values() and .entries() have not been implemented as:

• I don't see a use case,
• would be bad for performance,
• could be easily implemented by user via keys()