Make JvmStatic opt-in and fix an erroneous usage

[benjamin-bader]

All this time I've mistakenly believed that @JvmStatic converts a companion-object method into a bona-fide static method. That turns out to be untrue. What it does is add a second static method to the parent class that loads the companion object and calls the annotated method.

R8 optimizes this away, but not everybody uses R8. Since we're moving more and more to a Kotlin-only future, and in the service of (slightly) smaller code, this PR makes @JvmStatic opt-in only via a compiler flag.

It also fixes an erroneous usage of @JvmStatic, where @JvmField is the correct and intended annotation.

[codecov-io]

Codecov Report

Merging #417 (18f6db5) into master (84bd3f5) will decrease coverage by 0.04%.
The diff coverage is 11.11%.

@@             Coverage Diff              @@
##             master     #417      +/-   ##
============================================
- Coverage     59.71%   59.66%   -0.05%     
- Complexity      789      793       +4     
============================================
  Files            60       60              
  Lines          5488     5506      +18     
  Branches        860      866       +6     
============================================
+ Hits           3277     3285       +8     
- Misses         1967     1976       +9     
- Partials        244      245       +1     

Impacted Files	Coverage Δ	Complexity Δ
.../com/microsoft/thrifty/compiler/ThriftyCompiler.kt	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
.../com/microsoft/thrifty/kgen/KotlinCodeGenerator.kt	82.41% <25.00%> (-0.21%)	145.00 <0.00> (ø)
...main/kotlin/com/microsoft/thrifty/schema/Loader.kt	73.21% <0.00%> (+0.93%)	32.00% <0.00%> (+4.00%)

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