Move iteration over hash values from total_size() to sv_size()

Move iteration over hash values from total_size() to sv_size()

For now this requires some bodgery over when hash values should be recursed
into. Hashes presented to total_size() should be properly recursed into, else
the size of symbol tables isn't correctly reported. However, the back-link
from a CV to its stash shouldn't be traversed, as shouldn't GVs referenced from
OPs, else we run the risk of passing through %::, and adding the size of all
the symbol tables (and subroutines) to the size of a subroutine reference we
were directly passed. Whilst this might be useful in some cases, it isn't useful
as a default, and it isn't even vaguely consistent with the sizes total_size()
historically reported.