Use proper types for indices

Currently, we have many places in the code base which use int to represent indices. We should port them to enum classes as

• arithmetic on indices doesn't make sense (in general)
• separate indices should have separate types to avoid confusion; that is especially important given that we have some functions taking more than one index type
• using enums gives us a natural way to give invalid/default/other special states a name (instead of passing around magic numbers like -1 or INT_MAX)