0.19.0: Add perfs, laziness, and ascii art

• Have you see the new ascii art logo? It's pretty fast, too.
• Improve performance of switch substantially.
• Add fusion rules for skip, take, and slice. For example: stream.skip.take becomes stream.slice.
• Experimental Add map/slice commutation. For example, stream.map.slice.map becomes stream.slice.map.map, which is then fused into stream.slice.map
• Fix for recoverWith ending a stream too early.
• Breaking change concatMap now applies its mapping function lazily. See the docs for more information and visuals. This should only be a breaking change for code that has made assumptions about when the mapping function will be called.