Remember time bin names on serialization and restore on deserialization

Store the existing time bin names during serialization in the
attribute named "timeR_bin_name" of the srcref object (an INTSXP) and
restore the bin name during deserialization. To reduce the effect on R
programs, the attribute is deleted again during deserialization.

Known issues: Some functions are shown multiple times in the output,
with different execution counts.