Fix JNI code generation issues for Boolean return types, top-level functions, and visibility

[peacefulprogram]

1. Fix syntax error in generated JNI native functions when an @JniCall function returns Boolean.

3. Fix incorrect JNI native function names for top-level functions defined in lowercase-named files.
   The first character of the generated Java class name is always uppercase.

5. Make generated JVM external functions private, and preserve the visibility of generated JVM actual declarations.

[dshatz]

Hi thank you for the contribution!
The code looks fine!
Unfortunately I may not be able to merge this because of the big refactoring in #14. I will try to merge that this week we will see what can be done.

That branch also fixes some issues with top level functions and boolean conversion.
Maybe you can change the merge target to that branch?

[dshatz]

The first character of the generated Java class name is always uppercase.

I believe that's the default behavior when creating top level functions inside a file with non capitalized name.
demo.kt -> DemoKt