support for multiple return values & airy functions

[jorenham]

closes #17

• airy
• airye
• itairy
• airyb
• airyzo

[Copilot]

Pull Request Overview

This PR implements support for multiple return values and introduces Airy mathematical functions to the library. The key changes enable functions that return tuples of values rather than single values, with specific support for 4-tuple returns needed by the Airy function family.

Key Changes

• Added support for multiple return values in the build system and function generation
• Implemented five Airy functions: airy, airye, itairy, airyb, and airyzo
• Added comprehensive test infrastructure for 4-tuple return types (both real and complex)

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/lib.rs	Exports the new airy module functions
src/airy.rs	Implements all five Airy functions with trait-based polymorphism for real/complex inputs
build.rs	Extends code generation to support multiple return values and pointer-based C function calls
tests/test_functions.rs	Adds test infrastructure for 4-tuple return types and specific Airy function tests

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