Avoid infinite loop when unarchiving realsymbol and possymbol.

Avoid infinite loop when unarchiving realsymbol and possymbol.

symbol::read_archive(): explicitly set status_flags::evaluated (and
status_flags::expanded) on object being unarchived. These flags get
reset by basic::operator=(const basic&) for realsymbol and possymbol,
and nothing sets (except symbol ctor), so automatic evaluation never
terminates (or rather, terminates due to a stack overflow). Therefore
it's necessary need to set status_flags::evaluated explicitly.

Thanks to Markus Fröb for a bugreport and a test case.
(cherry picked from commit e99d0d58c1bbaa8ee73e4a90a90aa1086f2f813d)