Encoder improvements

[KillingSpark]

General improvements

• [ ] Add window size in frame header and stop using single segment for big frames (actually for raw/rle blocks a window size of 0 could be the correct value because no lookback is necessary for decoding?)
• [ ] Add support for RLE blocks by scanning the input for big-ish regions of the same byte
• [ ] Add support for actual compression using sequences (see #72 , this is a pretty big task)

Streaming support

• [ ] Allow feeding input piecewise and not require one contiguous slice
• [ ] Allow encoding into a slice piecewise and not require and growing a vec