Add standard library atomic operations to benchmark

[prashantv]

Currently, we only benchmark our custom atomic types, but don't
benchmark the standard library types. Add benchmarks for ints and uints
that the standard library supports.

Benchmark results on my machine:
https://gist.github.com/prashantv/4360013d6ec13b16ff983390e5defe73

There's no performance difference between stdlib and the atomic types (thanks to mid-stack inlining), e.g., with i32:

BenchmarkStress/i32/std/serial-8                50000000                41.7 ns/op                                    
BenchmarkStress/i32/serial-8                    30000000                41.2 ns/op                                    

BenchmarkStress/i32/std/parallel-8              10000000               174 ns/op                                      
BenchmarkStress/i32/parallel-8                  10000000               176 ns/op                                      

[codecov]

Codecov Report

Merging #35 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #35   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           2      2           
  Lines         118    118           
=====================================
  Hits          118    118

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[dorianperkins]

LGTM