Kotlin codegen

[benjamin-bader]

This PR implements Kotlin codegen of Thrift structures. It does everything except services, which need some more thinking. With that caveat, codegen itself is largely complete here, and just needs wiring up to thrifty-compiler to be usable from the command-line.

Still to-do before things are "shippable":

• services (obvs)
• wiring to thrifty-compiler
• rethinking compiler plugins for Kotlin (kotlinpoet is not interoperable with javapoet)

I feel comfortable merging this in the current state, without making it available in the command-line app.

[codecov-io]

Codecov Report

Merging #183 into master will decrease coverage by 0.08%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##             master    #183      +/-   ##
===========================================
- Coverage     72.48%   72.4%   -0.09%     
  Complexity      373     373              
===========================================
  Files            30      30              
  Lines          1730    1732       +2     
  Branches        165     165              
===========================================
  Hits           1254    1254              
- Misses          378     380       +2     
  Partials         98      98

Impacted Files	Coverage Δ	Complexity Δ
...crosoft/thrifty/compiler/TypeProcessorService.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...om/microsoft/thrifty/compiler/ThriftyCompiler.java	0% <0%> (ø)	0 <0> (ø)	⬇️

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