This patch makes summary use the excellent fenwick package from
https://github.com/yourbasic/fenwick to dramatically speed
up Add() calls while adding a bounded cost to memory.

    benchmark             old ns/op     new ns/op     delta
    BenchmarkAdd1-4       187           206           +10.16%
    BenchmarkAdd10-4      332           274           -17.47%
    BenchmarkAdd100-4     1092          325           -70.24%

The tree creation could be faster, but profiling has shown that
it doesn't seem to be that much of a problem (that's because
`summary.setAt()` happens a *lot* more than `summary.Add()`)