There might be duplicated enum values when allow_alias is true. Add PreferredAlias into OriginalNameAttribute to remove the duplication (#2727)

JSON tests fail, as we're not using OriginalNameAttribute yet.

Generated code changes for previous commit (basically InternalBuildGeneratedFileFrom => FromGeneratedCode)

Other changes are due to the well-known types changing without us regenerating.

We'll see what I've missed when CI fails...

Generated code changes from previous commit

Note that now we need a proto3 version of addressbook.proto. This may affect other platforms, and could do with an overhaul to follow proto3 conventions anyway (e.g. repeated field names). Will need to think about that carefully before merging into master. Raised issue #565 for this.

Prod code works, but some tests are broken. Obviously those need fixing, then more tests,
and review benchmarks.

(Primarily this is starting the hash code of messages at a non-zero value...)

Fixes issue #523.