Introduce `build.rs` to reduce boilerplate

[IvanIsCoding]

Follow up of #1252

Real diff: https://github.com/IvanIsCoding/rustworkx/compare/smarter-boilerplate...IvanIsCoding:rustworkx:build-rs

This PR leverages build.rs to reduce even more the boilerplate required to add a new rustworkx function. All needed now is to add mod your_module; to the main file after adding export_rustworkx_functions! to the new module.

I don't think we can generate multiple mod a; mod b; statments in Rust due to the limitations of include!. Still, I think this is worth keeping to make contributing more straightforward with some "magic".

[coveralls]

Pull Request Test Coverage Report for Build 10675943168

Details

• 83 of 84 (98.81%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 95.796%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
build.rs	76	77	98.7%
<!--	Total:	83	84	98.81%	-->

Totals
Change from base Build 10672676557:	-0.02%
Covered Lines:	17978
Relevant Lines:	18767

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💛 - Coveralls