[swift-generator] Plain JDK implementation of toString() for thrift structs

Currently, types generated by the generator depend on swift-codec (to handle the swift annotations), which means they indirectly depend on guava anyway. So this doesn't really buy us much right now.

It is part of my plan, though, to separate out the swift annotations so that a single version of a stub library containing just swift-generated interfaces and types could be usable across multiple versions of swift... and this version of toString() is easy enough, in case it would be useful to someone someday.