Need const versions of SvPV(), so that its callers can indicate

Need const versions of SvPV(), so that its callers can indicate
whether they are read/write or read only
Also provide a mutable version, for the internals to use where it
needs a non-const pointer, but will be writing. Callers of this are
assumed to be aware of how copy on write is working, and to keep
track of core changes.

For now check that SVs are not READONLY when const versions are used.
(with DEBUG_COW > 1). COW is going to have to be orthogonal to Perl's
READONLY flag, but for now it provides a good test indicator.

p4raw-id: //depot/perl@24721