allow protobuf messages to use a field named "size"

We ran into a bug where a protobuf with a field named "size" will have its encoded size miscalculated, because the computation uses a local var named "size". This fix just renames that local to "__size".

The vast majority of the diff is the re-generated scala files, since they all changed.

There is also one new test, which is pretty dumb: It just verifies that a protobuf with a "size" field has its size field computed correctly. Without this patch, the test throws an IOException. I won't feel bad if you erase the test. :)