constness for arrays

[Reiner Pope]

I like it. Same capabilities as I was grasping at, but much more elegant.

>   - reading and writing to the array has no additional cost
I just want to point out that this means that you can compile a release 
version with checking disabled for max speed, but for extra safety, you 
could set write-checking for debug builds, where the slowdown should be 
more acceptable.