Adds new model changes for Rust code generation

[desaikd]

Issue #136

Description of changes:

This PR adds new model changes for Rust code generation.

List of changes:

• Adds changes for Rust templates
  • Adds changes for using the new model
  • Adds changes for nested types to use template macros
• Adds changes for enabling Rust code generation
  • Add string_representation on FullyQualifiedTypeReference which will have namespace representated based on programming language
  • Modifies serializer for AbstractDataType to include ISL type name when serializing source IslType
• Add tests for Rust code generation

Generated code:

The generated code still stays the same only the template changes to use the new model.

Generated Rust code can be found here.

Tests:

Add roundtrip tests for Rust code.

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