ProtoReader32, a specialization for Kotlin/JS

[swankjesse]

Next steps:

• [x] Rename to ProtoReader32
• [x] Make it an interface with factory functions like fun ProtoReader32(ByteArray, Int, Int)
• [x] Create a function ProtoReader32.asProtoReader()
• [x] Make the base implementation of ProtoAdapter.decode(ProtoReader32) delegate by calling asProtoReader()
• [x] Make it opt-in to generate decode() functions that accept a ProtoReader32
• [x] Test all combinations of read functions