feat: Add binary tree functions.

[ianlewis]

Adds binary tree utility functions for dealing with binary syntax trees.

Fixes #51

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.68%. Comparing base (5233de2) to head (9386407).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #60      +/-   ##
==========================================
+ Coverage   88.46%   90.68%   +2.22%     
==========================================
  Files           3        3              
  Lines         338      365      +27     
==========================================
+ Hits          299      331      +32     
+ Misses         25       22       -3     
+ Partials       14       12       -2     

Files with missing lines	Coverage Δ
parser.go	93.46% <100.00%> (+3.78%)	⬆️

... and 1 file with indirect coverage changes

[ianlewis]

@superfrink I implemented some binary tree functions including rotations. This should hopefully make some use cases for parsing simpler, like the infix calculator.

[superfrink]

This error is not used anymore

var ErrMissingRequiredNode = errors.New("missing required node")

[superfrink]

Small increase in test coverage in superfrink@5e3df2c

[ianlewis]

This error is not used anymore

var ErrMissingRequiredNode = errors.New("missing required node")

Removed.

[ianlewis]

Small increase in test coverage in superfrink@5e3df2c

I merged this in. Small change is that I renamed the functions to TestNode_... since they are methods on the Node struct.

[superfrink]

Looks good to me. 👍