Major refactor to print_tree()

[jlumpe]

• Made formatting and structural changes to _print_tree() to simplify code and improve readability
• Added types to most arguments, explicit keyword arguments in public print_tree() function
• Removed unused to and from arguments to _print_tree()
• Typed TreeCharSet attributes as strings, constructor still accepts Chars. Added constructor which modifies select fields of an existing instance.
• Fix/implement index/key printing, rename withinds keyword argument to printkeys. Keys printed by default for return types of children() other than AbstractVector and Tuple.
• Fix bug where truncation character was printed under leaf nodes at max depth
• Test for multi-line node representations

[codecov-commenter]

Codecov Report

Merging #84 (fd91ba5) into master (e3afae7) will increase coverage by 3.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #84      +/-   ##
==========================================
+ Coverage   64.55%   67.58%   +3.03%     
==========================================
  Files           7        7              
  Lines         378      401      +23     
==========================================
+ Hits          244      271      +27     
+ Misses        134      130       -4     

Impacted Files	Coverage Δ
src/printing.jl	95.38% <100.00%> (+15.05%)	⬆️
src/iteration.jl	62.87% <0.00%> (-0.82%)	⬇️
src/AbstractTrees.jl
src/base.jl	100.00% <0.00%> (ø)
src/indexing.jl	19.60% <0.00%> (+1.24%)	⬆️
src/builtins.jl	66.66% <0.00%> (+6.66%)	⬆️

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