Add the ability to prune non-error entries out of the graph

[r-hang]

No description provided.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #218 into master will decrease coverage by 0.29%.
The diff coverage is 95%.

@@            Coverage Diff            @@
##           master     #218     +/-   ##
=========================================
- Coverage   98.93%   98.64%   -0.3%     
=========================================
  Files           9        9             
  Lines        1032     1104     +72     
=========================================
+ Hits         1021     1089     +68     
- Misses          9       11      +2     
- Partials        2        4      +2

Impacted Files	Coverage Δ
dig.go	98.73% <100%> (ø)	⬆️
internal/dot/graph.go	97.81% <94.93%> (-2.19%)	⬇️

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[abhinav]

Offline discussion, maybe something like this:

type ResultConsumer interface {
    // Asks the consumer to delete the reference to this Result.
    deleteResult(*Result)
}

// Ctor and Group implement ResultConsumer.

type Result struct {
    // ...
    
    Consumers []ResultConsumer
}