PERF: refactor tokenizer to give compiler branching hints. (OSX testers wanted)

Included vbench shows ~12% improvement. I've seen 15-20% on movielens, 8% on FEC.
I've encountered no regressions on any dataset I tried.

Tested with gcc 4.6,4.7,4.8 on linux, results from OSX+clang would be useful: 
`./test_perf.sh -H -r read_table_multiple_long` before and after.

Second VB commit is unrelated.

Will merge in a couple of days or so, if all is quiet.