Upload opaque pointer implementation of stack

[AvikAgarwala]

Description

Uploaded the opaque pointer implementation of stack ADT using C programming language. It consists of 3 files.

Motivation and Context

It helps to hide the implementation and member variables from the user

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.