New versions + stop using pre-release versioning

[madsmtm]

We're going to go away from using pre-release versions; while they're nice for signalling instability, they're a real pain when you need to do patch releases, see e.g. rust-windowing/winit#2739 (which is also why we skip objc2 v0.3).

While we release both objc2 v0.4 and block2 v0.2, these are not expected to be stable, and will still have frequent breaking changes. This is unfortunate, but unavoidable, and still better that we'd be able to patch these if e.g. winit requires it. I'll try to be harsher with objc2-encode v2 though, since users expect more stability from such a version number, but I will not let it severely hinder development.

Checklist:

• The branch is named new-versions, such that the full CI will run.
• Changelogs have only been modified under the Unreleased header.
• Version numbers are bumped in the following order:
  • objc-sys
  • objc2-encode
  • objc2
  • block2
  • icrate
• Local tests have been run (see helper-scripts/test-local.fish):
  • macOS 10.14.6 32bit
  • iOS 9.3.6, 1st generation iPad Mini
    • Am not at the same physical location, so will have to be skipped for now
• Any errors that emerge have been fixed in other PRs
  • Fix errors found during release testing #465

Post merge:

• A tag is created on the merge commit for each new version.