Refactor: Reduce clone of context object

[StoneDot]

Issue #, if available: This PR reduces the use of clone. There are primary and accompanying goals such as:

• Reduce clone to improve performance.
• Currently, there is not a good way to update context cache without quitting the program. It causes incoherent cache on shell mode. Mutable reference is a way to solve this problem.
• Give future ability into context to hold another cache like "item cache," which is required to implement completions.

This PR is related to #6.

I am welcome the discussion to achieve the above goals.

Description of changes:

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