re2: make tables const

Helps for moving tables to .rodata segment. Useful on chrome for Android.
http://code.google.com/p/chromium/issues/detail?id=249746
Also might help compiler optimisations.

R=rsc
CC=re2-dev
https://codereview.appspot.com/50220044

Committer: Russ Cox <rsc@swtch.com>