reduce allocation by 15%

[Moelf]

With the classical test:

julia> const t = LazyTree(ROOTFile("Run2012BC_DoubleMuParked_Muons.root"), "Events", ["Muon_pt"]);

julia> @benchmark let nmu=0
           for evt in t
               nmu += sum(evt.Muon_pt)
           end
           nmu
       end

Before:

BenchmarkTools.Trial: 1 sample with 1 evaluation.
 Single result which took 5.104 s (2.01% GC) to evaluate,
 with a memory estimate of 4.72 GiB, over 59724 allocations.

After:

BenchmarkTools.Trial: 2 samples with 1 evaluation.
 Range (min … max):  4.921 s …  4.931 s  ┊ GC (min … max): 1.24% … 1.29%
 Time  (median):     4.926 s             ┊ GC (median):    1.26%
 Time  (mean ± σ):   4.926 s ± 6.853 ms  ┊ GC (mean ± σ):  1.26% ± 0.04%

  █                                                      █  
  █▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁█ ▁
  4.92 s        Histogram: frequency by time        4.93 s <

 Memory estimate: 4.16 GiB, allocs estimate: 58155.

[codecov]

Codecov Report

Merging #94 (efa6650) into master (00d661a) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #94      +/-   ##
==========================================
- Coverage   84.84%   84.83%   -0.02%     
==========================================
  Files          11       11              
  Lines        1320     1319       -1     
==========================================
- Hits         1120     1119       -1     
  Misses        200      200              

Impacted Files	Coverage Δ
src/root.jl	90.86% <100.00%> (-0.05%)	⬇️

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