SGEC update, part 1

This is still an "experimental" feature, but it's getting expanded
a bit.  The implementation now lives in its own class.  An "export"
feature that generates SGEC data has been added.  The file extension
has been changed from ".sgec" to ".txt" to make it simpler to edit
under Windows.

SourceGen/SourceGen.csproj

@@ -79,6 +79,7 @@
     <Compile Include="FormattedOperandCache.cs" />
     <Compile Include="LocalVariableLookup.cs" />
     <Compile Include="MessageList.cs" />
+    <Compile Include="Sgec.cs" />
     <Compile Include="Tests\GenTest.cs" />
     <Compile Include="Tests\ProgressMessage.cs" />
     <Compile Include="Tests\WpfGui\GenTestRunner.xaml.cs">