Preserve child order after merge

[DrChainsaw]

Fixes #44

Just to not change up the tree more than necessary. Should have the same behaviour as Base.union w.r.t ordering now.

[codecov-io]

Codecov Report

Merging #46 (ef52be0) into master (fb582e3) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #46      +/-   ##
==========================================
- Coverage   87.01%   86.98%   -0.04%     
==========================================
  Files           6        6              
  Lines         362      361       -1     
==========================================
- Hits          315      314       -1     
  Misses         47       47              

Impacted Files	Coverage Δ
src/patterns.jl	84.09% <100.00%> (ø)
src/tree-ops.jl	82.85% <100.00%> (-0.17%)	⬇️

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