Introduce template classes for simple type rules (#2835)

Introduce template classes for simple type rules (#2835)

This commit introduces two template classes `SimpleTypeRule` and
`SimpleTypeRuleVar` to help define simple type rules without writing
lots of redundant code. The main advantages of this approach are:

- Less code
- More consistent error reporting
- Easier to extend type checking with other functionality (e.g. getting
the type of a symbol)