Introduces the `binary` USE flag (on by default) which allows
bootstrapping from `dev-lang/swift-bin` instead of building from source
using `dev-lang/swift`.
Swift 5.10 is becoming increassingly less desirable as a standalone
Swift version, and more difficult to build over time with newer
toolchain versions. For bootstrapping specifically, using a pre-built
binary is likely an acceptable default.
Signed-off-by: Itai Ferber <itai@itaiferber.net>
