Allow .proto3 file extensions

[nhynes]

Currently rust_protoc cannot compile onnx.proto3 but for no other reason than it ends with .proto3 instead of .proto. This PR remedies this by also trying to remove a .proto3 extension.

[stepancheg]

Thank you for the patch.

I think rust-protobuf should follow protoc behavior when generating code. When called for file named onnx.proto3, protoc generates:

For C++ it simply appends .h suffix:
onnx.proto3.pb.h for C++

For C# and Ruby it strips the suffix:
Onnx.cs for C#
onnx_pb.rb for Ruby:

For Java and Python it doesn't attempt to strip any suffix except .proto.

OnnxProto3.java for Java
onnx/proto3_pb2.py for Python

Anyway, my point is rust-protobuf should match behavior of Google's protobuf compiler:

• handle any file name suffix, not just .proto or .proto3
• stripped unknown suffix should be either appended to file name (as for Java) or ignored (as for C#) (I don't know what would be better)

If you wish to submit another patch, I would be happy to merge it. Note, patch should also include a test. Since it involves code generation based on file name, test should live somewhere inside protobuf-test directory.

[nhynes]

@stepancheg thanks for the comments! I took your advice to leave the unrecognized extension alone. Regarding testing, I updated the unit tests in strx.rs since that file contained the only non-trivial changes.

[stepancheg]

I guess with your new patch abc.proto3 will result in generated abc_proto3.rs which is good thing.

Test should be patched protobuf-test module. I can write a test myself, but it will take some time.

[nhynes]

I added unit tests to descriptorx.rs. I don't think that integration tests are necessary, actually.

[stepancheg]

Integration tests are necessary, so I had to write them myself.

Thank you for the patch! Merged. #268