log2n-span mergesort

[umutacar]

@jmgracecmu wrote a logn-span merge algorithm
https://github.com/jmgracecmu/gc-bench/tree/master/mergesort

would be nice to add this to the benchmark suite and compare

[shwestrick]

Cool! @jmgracecmu you could try comparing it against bench/msort which uses the lib/Mergesort implementation.