Drop support of key & value types other than string and Buffer about level-js HOT 2 CLOSED

We can't do it exactly the way I proposed in Level/memdown#186 (comment):

In level-js, store strings as (array)buffers

Because not all runtime environments support binary keys. In those we must use string keys only.

I'm thinking we'll replace:

With:

Level.prototype._serializeKey = function (key) {
  if (Level.binaryKeys) {
    return Buffer.isBuffer(key) ? key : Buffer.from(String(key))
  } else {
    return String(key)
  }
}

from level-js.

Also try to find a way to support reading/migrating existing data (that was stored as a string).

from level-js.