Implement ToBitMask trait also for types with 2 or 4 lanes

[jhorstmann]

A tiny followup to #239

[calebzulawski]

Actually, looks like #250 was nearly simultaneous and adds length-1 vectors too. I'm going to close this in favor of that one. Thanks for the contribution!

[workingjubilee]

Oh, sorryyyy. ^^;