Add support for accessing values from data-structures contained within `Const`

[kmr-srbh]

Overview

Trying to access a key from a Const dict threw a SemanticError stating type mismatch. An ASR pass error was thrown when trying to access values from a Const list, Const tuple and Const str. This was chiefly because the case of a Const value was not addressed in handling subscript indices.

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Fix

• Check for Const when visiting subscript indices and handle item access through the contained type in the llvm back-end.

• Throw a SemanticError for immutable types like tuple and str.

• Tuple

  

• String

  

• Dictionary

  

• List

  

Note

The get attribute of Const dict still throws a SemanticError now. I will push the fix for that in a separate PR.

[kmr-srbh]

@certik Please guide me where I should put the tests. It is a bit difficult to understand now. Is this the correct folder? Shall I refactor them to a more professional and readable manner?

[kmr-srbh]

The failing tests are irrelevant to the changes made.

[Thirumalai-Shaktivel]

Please mark this PR ready for review once it is ready.