Tweaks and more tests for maps

- Change the default message hash code to 1 to be consistent with other code
- Change the empty list/map hash code to 0 as "empty map" is equivalent to "no map"
- Removed map fields from unittest_proto3.proto
- Created map_unittest_proto3.proto which is like map_unittest.proto but proto3-only
- Fixed factory methods in FieldCodec highlighted by using all field types :)
- Added tests for map serialization:
  - Extra fields within entries
  - Entries with value then key
  - Non-contiguous entries for the same map
  - Multiple entries for the same key

Changes to generated code coming in next commit

csharp/generate_protos.sh

diff --git a/csharp/generate_protos.sh b/csharp/generate_protos.sh
index 60bc281..e861f16 100755
--- a/csharp/generate_protos.sh
+++ b/csharp/generate_protos.sh
@@ -43,10 +43,12 @@
 rm src/google/protobuf/descriptor_proto_file.proto
 
 $PROTOC -Isrc --csharp_out=csharp/src/ProtocolBuffers.Test/TestProtos \
+    src/google/protobuf/map_unittest_proto3.proto \
     src/google/protobuf/unittest_proto3.proto \
     src/google/protobuf/unittest_import_proto3.proto \
     src/google/protobuf/unittest_import_public_proto3.proto
 
+
 $PROTOC -Icsharp/protos/extest --csharp_out=csharp/src/ProtocolBuffers.Test/TestProtos \
     csharp/protos/extest/unittest_issues.proto