sv.c: Assert that sv_[ivp]v are not passed aggregates

The lack of assertions can hide bugs.  See 32a609747bffb for instance
toke.c:yylex: assert that PL_linestr is not a COW

and fix the one bug this uncovers.

With COW enabled all the time, it is easy to introduce bugs like this.
See also 4e917a04.  In short, toke.c expects to be able to modify
PL_linestr’s buffer.  Putting this assertion in yylex (a hot path)
will make sure PL_linestr never becomes a COW again.
op.c: Add op_folded to BASEOP

Add a new member, op_folded, to BASEOP.  It is replacement for
OPpCONST_FOLDED (which can only be set on OP_CONST).  At the moment
OPpCONST_FOLDED remains, as it is exposed in B (e.g. B::Concise relies
on it).

Signed-off-by: Niels Thykier <niels@thykier.net>
Add Niels Thykier to AUTHORS
more op_folded support: B, dump

also add more B::OP accessors for the missing bitfields