Updates to CLS

Featuring:
- New syntax element `id` to replace `variable_id|constant_id`
- Removing unused keyword `cast`
- Removing keyword `singleton-type`
- Updating singleton type section
-- TBD: update its title and remove the label for value type
- Updating section on type projections, adding examples
- Updating section on type designators, adding examples (hopefully correct ones)
- Typo fixes in function types section
- Extending section about existential types along with wildcard types
- Adding a note to dependent types
- Updating section on polymorphic method types, adding variance annotations
- Fixing type equivalence
- Large update on conformance section, more TBD
- Object definitions update (name may be lowercase, then not a constant)
- Updating section on nullability, removing controversial paragraph
- Fixing blocks section
- Updating statements section with `include` and `prepend` statements
- Fixing pattern matching