Non-functional miscellaneous improvements (#1445)

This commit contains non-functional (i.e. mostly stylistic, small
refactors, documentation and consistency) improvements that I've batched
and rolled up while working on #1342.

* Make `extraDependencies` from `InlineDependency` class private
* Miscellaneous improvements to `RustTypes.kt`
* Remove dead code from `RuntimeTypes.kt`
* Style change in `RuntimeTypes.kt`
* Miscellaneous improvements to `SymbolVisitor.kt`
* Miscellaneous improvements to `BuilderGenerator.kt`
    - Make it consistent with the `ServerBuilderGenerator` from #1342.
* Miscellaneous improvements to `TestHelpers.kt`
* Add docs to `headers_for_prefix` function
    - I keep rereading the implementation to see what this returns; it's
      not intuitive from the function name.