Complex API cleanup

There are two significant API changes to the Complex module in this release:

1. The unsafeLengthSquared property has been renamed lengthSquared, because--while it can overflow or underflow--the result is always well-defined, and therefore does not lead to memory unsafety. The old property is still present, marked unavailable, so you should get a useful renaming message from the compiler or your IDE. The old property will be removed after a period of time.

2. The Complex(length:phase:) initializer has been made non-optional. Instead it is now a precondition that the length must be either zero or infinite if phase is not finite. Updating code to account for this change should be quite straightforward, because you are unlikely to have been using the paths that could return nil previously. If you have any questions, please ask for assistance on the forums.

Linux testing improvements

Adds an empty Tests/LinuxMain.swift to fix building on Linux, and directs users to --enable-test-discovery for testing on Linux.

Initial tag

I haven't yet formally defined the release policy for this repo, but it's useful for SPM workflows to have a tag exist, so I'm cutting 0.0.0 with the first few rounds of bug fixes and cleanups from open source contributors.

Thanks to everyone who has jumped to contribute!