Investigate migrating `Complex` and other types to `swift-numerics`

[`swift-numerics`](https://github.com/apple/swift-numerics) is the official package for numerical types like `Complex` (and potentially `Matrix`?), so it would be nice to use it for better compatibility with the larger package ecosystem. Additionally, `swift-numerics` [provides helpers](https://github.com/apple/swift-numerics/blob/e30276bff2ff5ed80566fbdca49f50aa160b0e83/Sources/RealModule/ApproximateEquality.swift#L72) for determining approximate equality between floating points, which are the [recommended migration path](https://developer.apple.com/documentation/testing/migratingfromxctest) for `XCTAssertEqual(_:_:accuracy:)`. This should likely be done in a major version to avoid breaking clients.