Specify deterministic encoding for RLE columns

[alexjg]

From @ConradIrwin 's comments on #1

Currently changes are identified by the hash of their encoded content. There are a few places where that encoding is non-deterministic (we resolved LEBs already) but I didn't make any progress on specifying this for run length encoding (as an encoder can always chose between a literal run and a compressed run, we need to make sure all encoders make the same choice).