Add kotlin-codegen support for struct-valued constants

[benjamin-bader]

This is big enough that I'm leaving out more subtle questions of whether/how to enforce presence of optional fields or fields with default values, in deference to having a commit that is at least somewhat readable.

We'll settle that question prior to the next release.

[codecov-commenter]

Codecov Report

Merging #482 (7a77084) into master (d7d0998) will increase coverage by 0.25%.
The diff coverage is 68.57%.

@@             Coverage Diff              @@
##             master     #482      +/-   ##
============================================
+ Coverage     58.87%   59.12%   +0.25%     
- Complexity      864      865       +1     
============================================
  Files            69       69              
  Lines          6451     6481      +30     
  Branches       1004     1012       +8     
============================================
+ Hits           3798     3832      +34     
+ Misses         2360     2345      -15     
- Partials        293      304      +11     

Impacted Files	Coverage Δ
...in/kotlin/com/microsoft/thrifty/schema/Constant.kt	78.88% <42.85%> (+0.35%)	⬆️
.../com/microsoft/thrifty/kgen/KotlinCodeGenerator.kt	84.61% <75.00%> (+0.50%)	⬆️
.../kotlin/com/microsoft/thrifty/schema/StructType.kt	45.83% <0.00%> (+2.08%)	⬆️

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