Protogen rewrite; initial scratch can parse descriptor.proto; once we…

… have the emit basics, all of these types will be backported to use the generated versions

src/Directory.build.props

@@ -1,6 +1,6 @@
 <Project>
   <PropertyGroup>
-    <VersionPrefix>2.2.0</VersionPrefix>
+    <VersionPrefix>2.3.0</VersionPrefix>
     <RootNamespace>ProtoBuf</RootNamespace>
     <Authors>Marc Gravell</Authors>
     <OutputType>Library</OutputType>

src/protobuf-net.Test/Schemas/SchemaTests.cs

@@ -0,0 +1,43 @@
+using System.IO;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace ProtoBuf.Schemas
+{
+    public class SchemaTests
+    {
+        private ITestOutputHelper _output;
+        public SchemaTests(ITestOutputHelper output) => _output = output;
+        [Theory]
+        [InlineData(@"Schemas\descriptor.proto")]        
+        public void CanParse(string path)
+        {
+            Schema schema;
+            using (var proto = File.OpenText(path))
+            {
+                schema = Schema.Parse(proto);
+            }
+            foreach (var msg in schema.Messages)
+            {
+                WriteMessage(msg, 0);
+            }
+        }
+        private string Indent(int count) => new string(' ', count);
+        private void WriteMessage(Message msg, int indent)
+        {
+            _output.WriteLine($"{Indent(indent++)}{msg}");
+            foreach (var field in msg.Fields)
+            {
+                _output.WriteLine($"{Indent(indent)}{field}");
+            }
+            foreach (var res in msg.Reservations)
+            {               
+                _output.WriteLine($"{Indent(indent)}-{res}");
+            }
+            foreach (var subMsg in msg.Messages)
+            {
+                WriteMessage(subMsg, indent);
+            }
+        }
+    }
+}