Sweeping refactoring to improve error handling and code generation.

This release contains a large set of improvements led by @MrMage that improve error visibility and the structure of generated code. The Swift gRPC core now consists of two parts, a "Core" that provides basic services and a "Runtime" that contains code factored out of previously-generated code. With these changes, generated code is much lighter and easier to generate. To reduce dependencies, protoc-gen-swiftgrpc was rewritten to directly "print" generated code, and this simplification allowed us to move the plugin to the main Package.swift.

There are other pending improvements coming soon as patch version updates.