Building a big `bdsg::yomo::UniqueMappedPointer<bdsg::MappedIntVector>` and *then* saving it causes total chain size reported by `get_usage()` to increase

[adamnovak]

@xchang1 says that saving to a file bu filename/FD after building a big MappedIntVector in memory causes the object to be larger on disk and after load then it was before it was saved.