Implement log2 using integer-gmp

GHC.Integer.Logarithms provide a fast way to calculates the integer base 2 logarithm of a integer.

It's available since 7.2
http://www.haskell.org/ghc/docs/7.2-latest/html/libraries/integer-gmp-0.3.0.0/GHC-Integer-Logarithms.html