Migrate to go.elastic.co/fastjson

[axw]

• Remove internal/fastjson
• Use go.elastic.co/fastjson
• Update go.mod

Closes #228

[codecov-io]

Codecov Report

Merging #275 into master will decrease coverage by 0.83%.
The diff coverage is 47.25%.

@@            Coverage Diff             @@
##           master     #275      +/-   ##
==========================================
- Coverage    81.5%   80.66%   -0.84%     
==========================================
  Files          99       97       -2     
  Lines        5692     5644      -48     
==========================================
- Hits         4639     4553      -86     
+ Misses        832      827       -5     
- Partials      221      264      +43

Impacted Files	Coverage Δ
modelwriter.go	93.97% <ø> (ø)	⬆️
tracer.go	84.23% <ø> (ø)	⬆️
model/marshal_fastjson.go	74.46% <40.25%> (-10.57%)	⬇️
model/marshal.go	79.2% <85.71%> (+0.14%)	⬆️

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