modules/rle: consider offseting symbol count by one

per @grebe on #974 and #1006:

> I think it is useful to treat count as being offset by 1 so count=0 means 1 symbol, count=1 means 2 symbols, etc. Disallowing count=0 is wasteful when count is narrow, e.g. in the case of count being 1 bit it will always be longer. If you encode count with an offset, it should probably be named something like `offset_count` or `count_minus_one` to avoid confusion.