[WIP] Complete kotlinization

[samtstern]

In this PR:

• Write all missing Kotlin snippets
• Make file names more parallel
• Add a script to enforce snippet parity

[samtstern]

Woo it works! Build failed for missing snippet:

[samtstern]

@rosariopfernandes FYI I made a script to find any Java file that does not have a matching Kotlin file, or one where the Java file contains snippets that are not in the Kotlin file.

I think we should be able to delete all the interfaces after this, since we were only using them to enforce snippet parity. WDYT?

[thatfiredev]

Wow, that's awesome! Deleting the interfaces sounds good to me.

[samtstern]

Gonna merge this and then do a separate CL to figure out interfaces and fix the Kotlin code style.