Make Internals::SvREADONLY smarter

Make Internals::SvREADONLY smarter

(aka: More fun with Hash::Util)

$ perl -lMHash::Util=lock_value
$h{a} = __PACKAGE__; lock_value %h, a; $h{a} = "3"; print $h{a}'
^D
3

OK, so it didn’t really lock it. Now for more fun:

$ perl -lMHash::Util=unlock_value
$h{a} = __PACKAGE__; unlock_value %h, a; $h{a} =~ y/ia/ao/;
print __PACKAGE__
^D
moan

There are three different ways to fix this:
1) Add an SvFAKE function to Internals:: (not *more* ‘internals’ for
   people [ahem, Const::Fast, ahem] to abuse!)
2) Use B::* functions in Hash::Util to check the flags (too slow)
3) Make Internals::SvREADONLY less ‘internal’, by having it deal with
   readonliness in general, rather than just the SVf_READONLY flag.

The third approach seems the most logical, so that’s what this
commit does.

There is one test in t/op/tr.t that uses Internals::SvREADONLY to
detect bovinity, so I’ve changed it to use B instead, as that will
have no effect on post-install efficiency.

(This approach also fixes Const::Fast’s bugginess, but that is purely
accidental.)