Simplify and optimize value extractors

Speed up results by Joona:

gcc-4.7 (1.5%)
gcc-4.8 (0.5%)
gcc-4.9 (1.0%)

Speed up results by mstembera:

gcc 474
p-value: 0.719

gcc 482
p-value: 1

gcc 492
p-value: 0.859

No functional change

Resolves #211