Implement abs intrinsic function

[namannimmo10]

No description provided.

[certik]

This is fine for now. Just remember this is only the compile time optimization. We will keep this even after we implement an actual abs function in the runtime library, and the compile time value will be provided in the value member.

Also, the current implementation is replacing abs with the compile time value. Ok for now, but the correct solution is to represent it using the abs runtime function, and then provide compile time evaluation (if available) in the value member of the FunctionCall node.

[certik]

Also, would you please mind sending it from a branch in your fork, not in the lcompilers/lpython repository? That way we only have the main branch in the main repository.

[namannimmo10]

Sure, I will make sure that will not happen again.