Consistently resolve transitive substitutions

[rtphubeny]

The existing implementation of substitution resolution inconsistently resolves transitive substitutions of Object contents because the Object is an unordered map. If a dependent is evaluated before the dependency, the dependent is not properly resolved.

For example:

{
  a: 5,
  b: ${a},
  c: ${b},
}

c will correctly resolve to 5 if b has been resolved first. However, if c is resolved first, c is evaluated to ${a}.

According to HOCON docs,

In general, in resolving a substitution the implementation must:

• lazy-evaluate the substitution target so there's no
  "circularity by side effect"
• "look forward" and use the final value for the path
  specified in the substitution
• if a cycle results, the implementation must "look back"
  in the merge stack to try to resolve the cycle
• if neither lazy evaluation nor "looking only backward" resolves
  a cycle, the substitution is missing which is an error unless
  the ${?foo} optional-substitution syntax was used.

This pull request does not specifically implement a "forward" or "backward looking" solution, but it does evaluate the graph for cycles and improves substitution resolution consistency.

[codecov]

Codecov Report

Base: 95.34% // Head: 95.39% // Increases project coverage by +0.05% 🎉

Coverage data is based on head (e6dc29c) compared to base (67c3b38).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #37      +/-   ##
==========================================
+ Coverage   95.34%   95.39%   +0.05%     
==========================================
  Files           3        3              
  Lines         773      782       +9     
==========================================
+ Hits          737      746       +9     
  Misses         24       24              
  Partials       12       12              

Impacted Files	Coverage Δ
parser.go	95.88% <100.00%> (+0.07%)	⬆️

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[rebecca-hubeny-okcupid]

@gurkankaymak can you please take a look at this pull request when you have the time?