Remove the RleDecoder::seek() function.

Note that there are still corresponding seek functions in classes `BooleanRleDecoderImpl`, `ByteRleDecoder`.

I changed `RleDecoderV1` to depend upon `google::protobuf::io::ZeroCopyInputStream`—minus the seeking capability added by `SeekableInputStream`—as its stream type, but it never decodes any Protocol Buffer types from this stream, so there's no reason to rely on such a non-standard stream interface. Again, I'd prefer to use `std::basic_istream`.

This proposal fixes #17.