improve performance

I noticed after going from 0.3.1 to 0.3.2, that performance of random and pseudo modes dropped by 50%!  It looks like readable-stream 1.1.0 is to blame.

mode | 0.3.1 | 0.3.2 | PR
------------------ | ----------- | ------------ | -----------
random           | 400Mbps | 197Mbps | 453Mbps
pseudo           | 382Mbps | 202Mbps | 451Mbps
0                     | 845Mbps | 810Mbps | 822Mbps
alpha              | --             | 263Mbps | 256Mbps
num                | --             | 244Mbps | 241Mbps
pregenerated | --             | 785Mbps | 845Mbps

I don't know why, and haven't looked around for readable-stream issues that may be related, but just dropping back from 1.1.0 to 1.0.x fixes it.  I also tweaked the pregenerated mode for speed, but it comes at the cost of being vulnerable to downstream consumers writing to the returned data.  For that reason, I left both behaviors in there, with only the faster behavior active.