Leverage Base reduction functions for sum

[asinghvi17]

Re-implementation of #7 with some extra tests. I could use Kahan summation in some GeometryOps.jl things I am doing, especially the sum_kbn(f, itr) form.

# this implementation
julia> @be sum_kbn($(rand(1000)))
Benchmark: 3348 samples with 16 evaluations
 min    1.677 μs
 median 1.734 μs
 mean   1.738 μs
 max    7.698 μs

# the implementation on master
julia> @be KahanSummation.sum_kbn($(rand(1000)))
Benchmark: 3336 samples with 16 evaluations
 min    1.682 μs
 median 1.698 μs
 mean   1.740 μs
 max    4.336 μs

# compared to current Base pairwise summation, slower of course
julia> @be sum($(rand(1000)))
Benchmark: 3198 samples with 290 evaluations
 min    98.131 ns
 median 98.707 ns
 mean   100.700 ns
 max    268.390 ns

I attached a timer output and it never calls the method for two TwicePrecisions. (Annotations mine)

julia> to
────────────────────────────────────────────────────────────────────
                           Time                    Allocations      
                  ───────────────────────   ────────────────────────
Tot / % measured:      2.13s /   0.8%           3.68MiB /   0.0%    

Section                           ncalls     time    %tot     avg     alloc    %tot      avg
───────────────────────────────────────────────────────────────────────────────
+(::Float64, ::TwicePrecisionN)     1.72M   17.9ms   99.9%  10.4ns     0.00B     - %    0.00B
+(::Float64, ::Float64)             1.72k   17.4μs    0.1%  10.1ns     0.00B     - %    0.00B
───────────────────────────────────────────────────────────────────────────────

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (8d24619) to head (c659cf8).

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #37   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           56        64    +8     
=========================================
+ Hits            56        64    +8     

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[JeffreySarnoff]

@simonbyrne any objection?