Revert "Implement the bipolar read-only system"

This reverts commit a623f8939cbcaa58a069807591675c0ebcd4135b.
There's no need to waste the last free SV bit needed for SVf_TYPED
for a hard READONLY flag. protecting Hash::Util::unlock from unlocking
protected SVs is trivial to do, even in pure-perl.
Revert "Add SVf_PROTECT"

This reverts commit fd01b4b766a3276a9439cade9b1a047c37876c1b.
ensure Hash::Util::unlock* does not unprotect protected values

Check explicitly for B::SPECIAL values, and do not SvREADONLY_off those