Add ability to `flush` an `Encoder.Feed`

[05nelsonm]

When using an Encoder.Feed directly, it'd be nice to be able to call doFinal w/o closing the feed so that multiple things can be encoded without having to instantiate a new feed. Having the ability to call flush will open up the possibility to perform multiple encodings within the .use lambda.

val sb = StringBuilder()
Base64().newEncoderFeed { encodedChar ->
    sb.append(encodedChar)
}.use { feed ->
    messageBytes.forEach { b -> feed.consume(b) }

    feed.flush()

    sb.append(':')

    signatureBytes.forEach { b -> feed.consume(b) }
}

[05nelsonm]

flush() should be available for both Encoder.Feed and Decoder.Feed.

Because of some of the implementations that hold on to some stateful data, this should be implemented in the following manner:

• Add EncoderDecoder.Feed.flush
• Add EncoderDecoder.Feed.flushProtected
• Encoder.Feed and Decoder.Feed use final override flush and call flushProtected

Implementors of EncoderDecoder.Feed were required to OptIn to the ExperimentalEncodingApi, so this is a "non-breaking" change

[05nelsonm]

Actually, this can be done w/o the need to break the current API