Add basic class support:

 * frontend API extended to allow converting an entire class
 * function conversion now supports object methods
 * responsibility for renaming the top level object is now moved up from
the call tree transformer - this is because functions may or may not be renamed based on context (e.g. object members are not renamed)
Not yet implemented:
 * static analysis and side effect protection for object fields

PiperOrigin-RevId: 182423203