improve a couple benchmark implementations

[vtjnash]

on master

julia> run(BaseBenchmarks.SUITE[["shootout", "binary_trees"]])
BenchmarkTools.Trial: 
  memory estimate:  13.96 MiB
  allocs estimate:  783717
  --------------
  minimum time:     13.063 ms (0.00% GC)
  median time:      14.136 ms (0.00% GC)
  mean time:        14.747 ms (4.12% GC)
  maximum time:     27.875 ms (0.00% GC)
  --------------
  samples:          340
  evals/sample:     1

julia> run(BaseBenchmarks.SUITE[["shootout", "regex_dna"]])
BenchmarkTools.Trial: 
  memory estimate:  1.69 MiB
  allocs estimate:  211
  --------------
  minimum time:     3.762 ms (0.00% GC)
  median time:      4.582 ms (0.00% GC)
  mean time:        4.772 ms (3.45% GC)
  maximum time:     10.446 ms (52.77% GC)
  --------------
  samples:          1048
  evals/sample:     1

julia> run(BaseBenchmarks.SUITE[["problem", "json", "parse_json"]])
BenchmarkTools.Trial: 
  memory estimate:  342.91 KiB
  allocs estimate:  13741
  --------------
  minimum time:     510.597 μs (0.00% GC)
  median time:      560.140 μs (0.00% GC)
  mean time:        599.678 μs (2.75% GC)
  maximum time:     3.195 ms (81.09% GC)
  --------------
  samples:          8318
  evals/sample:     1

on PR

julia> BaseBenchmarks.run(BaseBenchmarks.SUITE[["shootout", "binary_trees"]])
BenchmarkTools.Trial: 
  memory estimate:  8.02 MiB
  allocs estimate:  262842
  --------------
  minimum time:     2.939 ms (0.00% GC)
  median time:      3.312 ms (0.00% GC)
  mean time:        3.667 ms (8.86% GC)
  maximum time:     7.807 ms (38.13% GC)
  --------------
  samples:          1364
  evals/sample:     1

julia> run(BaseBenchmarks.SUITE[["shootout", "regex_dna"]])
BenchmarkTools.Trial: 
  memory estimate:  588.95 KiB
  allocs estimate:  298
  --------------
  minimum time:     3.176 ms (0.00% GC)
  median time:      3.651 ms (0.00% GC)
  mean time:        3.776 ms (1.29% GC)
  maximum time:     28.847 ms (0.00% GC)
  --------------
  samples:          1324
  evals/sample:     1

# Additionally with https://github.com/JuliaLang/julia/pull/40484
#julia> BaseBenchmarks.run(BaseBenchmarks.SUITE[["shootout", "regex_dna"]])
#BenchmarkTools.Trial: 
#  memory estimate:  582.50 KiB
#  allocs estimate:  174
#  --------------
#  minimum time:     2.719 ms (0.00% GC)
#  median time:      3.160 ms (0.00% GC)
#  mean time:        3.268 ms (1.66% GC)
#  maximum time:     8.119 ms (60.51% GC)
#  --------------
#  samples:          1530
#  evals/sample:     1

julia> run(BaseBenchmarks.SUITE[["problem", "json", "parse_json"]])
BenchmarkTools.Trial: 
  memory estimate:  159.77 KiB
  allocs estimate:  7659
  --------------
  minimum time:     335.786 μs (0.00% GC)
  median time:      367.212 μs (0.00% GC)
  mean time:        404.287 μs (1.23% GC)
  maximum time:     151.886 ms (0.00% GC)
  --------------
  samples:          10000
  evals/sample:     1