Use StaticArrays instead of Vector in RuleNode

[THinnerichs]

StaticArrays are particularly useful whenever the size is already known. As we never CHANGE a RuleNode, but only substitute the children 1. the RuleNode struct can and should be immutable, and 2. the number of children stays constant. This allows us to use SizedArrays (not MVectors, as for mutation the eltype must be isbitstype (i.e. no references, elements immutable, ... ).

References Herb-AI/Herb.jl#148

[codecov-commenter]

Codecov Report

❌ Patch coverage is 66.66667% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.41%. Comparing base (ebf0001) to head (a47150d).
⚠️ Report is 62 commits behind head on master.

Files with missing lines	Patch %	Lines
src/rulenode.jl	66.66%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #43      +/-   ##
==========================================
+ Coverage   75.36%   76.41%   +1.04%     
==========================================
  Files           2        2              
  Lines         203      212       +9     
==========================================
+ Hits          153      162       +9     
  Misses         50       50              

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