Remove get magic from typeglobs. This means that PVGVs holding

Remove get magic from typeglobs. This means that PVGVs holding
typeglobs never need to use SvPVX. This comes at price - typeglobs
were using magic get for their stringificiation, and to pass SvOK(),
so need to make typeglobs SvOK by default (by sucking SVp_SCREAM into
SVf_OK - it's the only flag left), tweak SvSCREAM() to also check
SVp_POK, and teach sv_2[inpu]v how to convert globs.
However, it should free up SvPVX for the next part of the plan to
pointer indirections, and therefore CPU cache pressure.

p4raw-id: //depot/perl@27278