Fix cycle reference during serialization.

[hs-lsong]

And also remove some un-used fields.

cc @liamrharwood

[codecov-io]

Codecov Report

Merging #319 into master will decrease coverage by 0.1%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #319      +/-   ##
============================================
- Coverage     71.43%   71.32%   -0.11%     
+ Complexity     1562     1558       -4     
============================================
  Files           239      239              
  Lines          4926     4918       -8     
  Branches        793      793              
============================================
- Hits           3519     3508      -11     
- Misses         1124     1126       +2     
- Partials        283      284       +1

Impacted Files	Coverage Δ	Complexity Δ
...java/com/hubspot/jinjava/lib/fn/MacroFunction.java	100% <ø> (ø)	9 <0> (-3)	⬇️
...main/java/com/hubspot/jinjava/lib/tag/CallTag.java	100% <100%> (ø)	3 <0> (ø)	⬇️
...ain/java/com/hubspot/jinjava/lib/tag/MacroTag.java	82.75% <100%> (-1.62%)	8 <0> (ø)
...in/java/com/hubspot/jinjava/lib/tag/ImportTag.java	80.95% <0%> (-7.15%)	7% <0%> (-1%)

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