Refactor: Extract duplicated code patterns into reusable helpers

[Copilot]

Three areas of code duplication were eliminated through extraction to helper functions:

Changes

• sexp.rs: Extracted wrap_tokens_with_special_chars() from two identical 23-line blocks that wrapped tokens with quote/backtick/comma patterns in expand_special_characters()

• eval.rs: Extracted eval_statements_sequentially() from duplicate logic in Let and Progn that iterated statements and returned the last evaluation result

• eval.rs: Extracted eval_binary_arithmetic_op() and eval_binary_comparison_op() from repetitive arithmetic (+, -, *, /, %) and comparison (<, >) operation handlers

Example

Before:

"-" => Rc::new(Sexp::Num(
    eval_sexp_to_num(l[1].clone(), state) - eval_sexp_to_num(l[2].clone(), state),
)),
"+" => Rc::new(Sexp::Num(
    eval_sexp_to_num(l[1].clone(), state) + eval_sexp_to_num(l[2].clone(), state),
)),

After:

"-" => eval_binary_arithmetic_op(l, state, |a, b| a - b),
"+" => eval_binary_arithmetic_op(l, state, |a, b| a + b),

Net reduction: 12 lines of code, with improved maintainability through reusable abstractions.

Original prompt

Find and refactor duplicated code

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