Implement run-length-encoding in Simple-8b

In an unused Simple-8b selector slot (number 15) let the remaining 4 bits of the first byte in a Simple-8b block represent a RLE count. 

This count should be treated as a repeat of the previous value multiplied with 120.

The encoder may have to look back several written Simple-8b blocks and re-write as RLE.

A repeat of 119 needs the following Simple-8b blocks:

#1 <60 buckets>
#2 <30 buckets>
#3 <20 buckets>
#7 <8 buckets>
#14 <1 bucket>

When the 120th identical value is appended they can all be replaced by RLE selector #15 with count=1.