Atomics 1.0.0

This release marks an important milestone for this package -- the advent of source stability!

Changes

• The public API of Swift Atomics is now considered source stable. The README defines what we consider public API.
• Swift Atomics now requires a Swift 5.3 or later toolchain.
• The low-level (pointer-based) atomic boolean operations are now public. (They were accidentally omitted from previous releases.)
• The package now includes a CMake-based build method for bootstrapping purposes. This isn't considered stable; it may be removed or arbitrarily changed in any new release, including patch releases.

Pull requests

• #20 DoubleWord._readersBitWidth: Switch to using Int.bitWidth (by @lorentey)
• #24 Mark the additional Bool methods as public (by @dfunckt)
• #25 build: add a CMake based build for Atomics (by @compnerd)
• #26 _AtomicShims: simplify double word setup (by @compnerd)
• #28 gitignore: ignore vim swap files (by @compnerd)
• #29 Tests: add XCTest manifest for non-SPM based builds (by @compnerd)
• #30 Bump required toolchain version to 5.3 (by @lorentey)
• #31 Declare Atomics source stable and describe what we consider public API (by @lorentey)
• #32 [NFC] Update copyright notices (by @lorentey)

Thank you to everyone who contributed to this release!