SwiftProtobuf 1.7.0 Release

Complete support for Swift 5.1.

Additions to the library:

• The generated code for descriptor.proto is now included in the library, this means one no longer has to generate and compile it in if there are proto files that had extension declared on those types. (#727)

Notable changes:

• The Swift 5.1 compiler will error on switch statements for an enum if it has a large number of cases; this could happen in the generated code. The generated code now splits things up to avoid this compile error. (#904)