Expands "compact" row.names during deserialization

GNU R employs "compact" row.names in the form c(-n, NA)
to avoid allocating character vectors in the form 1..n.

Because this requires a proliferation of special
case handling throughout the core, Renjin uses
instead a virtual sequence vector, which requires no
allocation.

However, when reading in .rds files etc possibly generated by GNU R,
we need to expand these compact vectors to their full form.