Reimplement java.io.Bits using VarHandle access

Currently, `java.io.Bits` is using explicit logic to read/write various primitive types to/from byte arrays. Switching to the use of VarHandle access would provide better performance and less code.

Also, using a standard API for these conversions means future VarHandle improvements will benefit Bits too.

Improvements in Bits will propagate to ObjectInputStream, ObjectOutputStream and RandomAccessFile.