rename core classes

[andrewparmet]

• Get rid of kt prefixes
• messageSize is a function so it can't conflict with a field message_size
• We had three "serializer" names: KtDeserializer, KtMessageDeserializer, and KtMessageSerializer. The second and third are mostly internal. I renamed them Writer and Encoder to try to highlight that, but I don't love that name. I just can't think of better. Open to suggestions. Other options are Writer and Reader; pb-and-k uses Marshaller and Unmarshaller; Javaland's equivalents are the coded streams. used Reader and Writer

[andrewparmet]

https://github.com/protocolbuffers/protobuf/blob/main/java/kotlin/src/main/kotlin/com/google/protobuf/Anies.kt

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