Add support for accessing values from `Const`

[kmr-srbh]

Overview

Currently, trying to access a key from a Const dict throws a semantic error stating type mismatch. An ASR pass error is thrown when trying to access values from a Const list. This was chiefly because the case of a Const value was not addressed in handling subscript indices. The error exists for Const types in general and will be addressed in subsequent commits.

Fix

This PR checks for Const when visiting subscript indices and handles access through the contained type.

Prerequisite

This PR depends on accessing the expr_t of the contained type inside Const. I have tried high and low but could not work out a fix. The variable in question is ASR::expr_t* value which is passed to visit_SubscriptIndices.

The issue is:

• Consider a dictionary declaration: Const[dict[str, i32]] = {"a": 1, "b": 2}.
• value contains Const[dict[str, i32]].

We need value to get to store dict[str, i32].

[kmr-srbh]

The errors are okay because of the problem with value variable. See above.