Adds changes for core isl types

[desaikd]

Issue #13

Description of changes: This PR works on implementing ISL core types

Changes:

• adds CoreIslType and CoreISLTypeDefinition for lob, number, text, any core types implementation
• renames TypeDefintionImpl method parse_from_isl_type_and_update_pending_types
• adds changes to TypeStore, PedningTypes to store core types
• adds unit tests for this change
• core_types.rs stores Hashmap of ISL core types and their definition

Test: adds tests for ISL core types

[desaikd]

Commit changes for https://github.com/amzn/ion-schema-rust/pull/59/commits/099f7070bf66a9db2be12e0e12a1d57b03af7b67:

• Removed CoreISLType enum from the ISL internal representation layer
• Removed enum variant CoreIsl from IslTypeRef, Named(String) enum varint will now represent both aliases and builtin types
• Renamed CoreIslTpeDeifnition to be BuitlInTypeDefinition
• Atomic(IonType, bool) enum variant represents all ion types and all '$' prefixed ion types where bool represents whether they allow null or not
• Derived(TypeDefinitionImpl) enum variant represents all derived ISL types like any, number, text, lob
• All the BuiltInTypeDefinition(Atomic + Derived) will be preloaded to type_store (type_ids: 0-31)
• Resolved bug: changed PendingTypes#get_total_types() to include type_store length for correct type_id calculation
• TypeStore#new() now returns an IonSchemaResult<Self> due to preload()
• Adds more validation and load_schema tests for buitlin types
• Includes InvalidNull enum variant for Violations