Refactors the implementation and the test

[Asterless]

This pull request refactors and modernizes both the implementation and the test suite of the reactive library, focusing on code style improvements and enhanced test assertions. The main changes include replacing manual increment/decrement operations with the += and -= operators, updating test assertions to use more idiomatic assertion functions, and improving output formatting for example and test code.

Operator Implementation Improvements

• Replaced manual increment/decrement operations (e.g., count = count + 1) with compound assignment operators (count += 1, count -= 1) in all observable operators for consistency and clarity. (src/reactivex.mbt)

Test Suite Modernization

• Updated all test assertions to use idiomatic assertion functions (assert_eq, assert_true, assert_false) instead of manual checks and panic(), improving readability and reliability of tests. (src/test.mbt)

Output Formatting Enhancements

• Changed string interpolation to explicit concatenation and value conversion using value.to_string() in all example and test output, ensuring correct and consistent output formatting. (src/examples.mbt, src/test.mbt)

These changes collectively improve code readability, maintainability, and test reliability across the codebase.