Drop 0.x from the version number

[dphfox]

Right now, Fusion versions are numbered 0.1, 0.2, 0.3, etc. This is meant to demonstrate that Fusion is still exploring different designs, however it leads to confusion about the project's level of polish as it suggests kinds of instability that we actively work to avoid.

Plus, with 0.3, a whole host of fundamental problems are now solved and completely correct Fusion programs are now possible to express, so it seems like we would be able to build atop it easily into the future.

So I'd like to suggest that, to reduce confusion, perhaps we should be representing these changes with major version numbers instead. Fusion 0.3 would become Fusion 3, and the next major feature release would become Fusion 4, etc.

For the sake of clarity, Fusion versions that are already released would remain 0.2 and 0.1.

[dphfox]

Discussed this on Discord. Instead of renaming 0.3, it's probably a better idea to split it so that everything that's done so far is 0.3 and everything afterwards is Fusion 1.