adds changes for anonymous and named `TypeDefinition`s and `Isl*` implementations

[desaikd]

Issue: Fixes #25

Description of changes:

• adds IslType enum with two different implementations for anonymous and named ISL types
• adds TypeDefinition enum with two implementations for anonymous and named type definitions
• Fixes a bug in PendingTypes to store correct TypeId in Constraints (type_id + type_store.types_by_id.len())

Tests: adds changes for using corresponding anonymous/named types in unit tests.