adds `TypeReference` implementation and `$null_or` ISL 2.0 modifier support

[desaikd]

Issue #164, #158

Description of changes:

This PR works on adding $null_or modifier support for ISL 2.0 and adds an implementation of TypeReference.

List of changes:

• add implementation of IslTypeReferenceModifier which stores modifier information for given type reference
• add utility methods to create nullable_built_in_type_ref and `null_or_named_type_ref.
• only allows built in types to use nullable modifier for ISL 1.0
• adds implementation of TypeReference which stores type id and modifier information for givne type reference
• all constraints now uses TypeReference instead of TypeIds to store type reference information
• adds document type to defintioon of $any
• adds validate_for_base_nullable_type to get the underlying ion type for a type definition
• adds default type $any for ISL 2.0
• adds unit tests for $null_or modifier
• renames TypeDefinition to TypeDefinitionKind

• renames TypeRef to TypeDefinition

  By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[desaikd]

Updates after last review:

• adds suggested changes

  • renames IslTypeRefModifier to NullabilityModifier
  • adds doc comment changes
  • changes default type addition to be only applied for ISL 1.0
  • changes nfa to use TypeReference instead of TypeId to consider nullability
  • $null_or modifier should not be allowed on a type reference that has occurs field in its definition

• adds implementation of VariablyOccurringTypeRef

  • removes unused method TypeDefinitionKind::get_occurs_constraint() now that we have VariablyOccurringTypeRef
  • modifies IslTypeRefImpl::get_occurs_constraint() to return Option<Range> instead of bool
  • renames IslTypeRefImpl::get_occurs_constraint() to IslTypeRefImpl::get_occurs_range()
  • removes implementation of OccursConstraint as it is treated as no-op and is handled by VariablyOccurringTypeRef