- Custom pattern matching through unapply functions.
- TypeParameter now has private constructor, can only be created
  with TypeParameter.fresh or _.freshen
- Stricter type checking for expressions. Correct various type errors.
- Remove some redundant code.

Methods can be overriden in subclasses
Non-root classes can define methods
Deep hierarchies are supported

Companion objects to lift methods
Remove standalone objects, they become isPackageObject

Refactor all existing Modulo into Remainder, as it is
the correct term in Scala for the operation %.

BigInt now supports `mod` operator, which gets represented
as Modulo in Leon.

In order to have support for maps, you need to import leon.lang._