Drop support of key & value types other than string and Buffer

[vweevers]

Background: Level/memdown#186.

[vweevers]

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:

level-js/index.js

Lines 143 to 151 in 86dd5e7

	Level.prototype._serializeKey = function (key) {
	if (Buffer.isBuffer(key)) {
	return Level.binaryKeys ? key : key.toString()
	} else if (Array.isArray(key)) {
	return Level.arrayKeys ? key.map(this._serializeKey, this) : String(key)
	} else {
	return key
	}
	}

With:

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

[vweevers]

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