Swagger 3 support

[yonaskolb]

This updates from Swagger 2.0 to OpenAPI/Swagger 3.0 https://swagger.io/specification
Resolves #47

This includes updating the:

• Swagger parser
• SwagGen generator
• Swift template

Note that this initial PR doesn't include all the new Swagger 3 features, but just updates everything to get it working.

Proper support for the following is still missing:

• Headers
• Examples
• Multiple media types
• Links
• Callbacks
• Custom encoding
• Discriminators with oneOf and anyOf
• External documentation
• XML

Note also that this drops compatibility for Swagger 2.0

[cooksimo]

I've just tried to use this and SPM is reporting unresolvable dependencies for me.

[yonaskolb]

@cooksimo are you using this package as a dependency or trying to build this on its own? Does it say which dependencies are unresolvable?

[cooksimo]

Was attempting to use it as a dependency.

Package.swift contents:

dependencies: [
        // Dependencies declare other packages that this package depends on.
        .package(url: "https://github.com/yonaskolb/SwagGen.git", .branch("swagger_3")),
    ],

swift package generate-xcodeproj output:

Updating https://github.com/onevcat/Rainbow.git
Updating https://github.com/yonaskolb/SwagGen.git
Updating https://github.com/yonaskolb/JSONUtilities.git
Updating https://github.com/kylef/Spectre.git
Updating https://github.com/yonaskolb/Stencil.git
Updating https://github.com/jpsim/Yams.git
Updating https://github.com/kylef/Commander.git
Updating https://github.com/kylef/PathKit.git
Updating https://github.com/SwiftGen/StencilSwiftKit.git
Updating https://github.com/jakeheis/SwiftCLI
error: dependency graph is unresolvable; found these conflicting requirements:

Dependencies:
    https://github.com/yonaskolb/SwagGen.git @ swagger_3
warning: PackageDescription API v3 is deprecated and will be removed in the future; used by package(s): JSONUtilities, Stencil

EDIT
Ignore me, I ran swift package reset and then deleted the Package.resolved and now it seems to be working again. Will try it out!

[azimin]

Checked with my small spec migration, works fine. Will report if we would face anything.
Thanks @yonaskolb

[yonaskolb]

Merging this into master. Followup PR's will add more features