Revert "[perl #119043] Exempt shared hash key consts from ro"

This reverts commit ba36554e02872e48d146177a57a9cfb154727fae.

It turns out it reinstates bugs like this:

$ perl -e 'for(1..10){for(__PACKAGE__){warn $_; $_++}}'
main at -e line 1.
maio at -e line 1.
maip at -e line 1.
maiq at -e line 1.
mair at -e line 1.
mais at -e line 1.
mait at -e line 1.
maiu at -e line 1.
maiv at -e line 1.
maiw at -e line 1.
Test that __PACKAGE__ is read-only

I.e., test that this bug is fixed:

$ perl -e 'for(1..10){for(__PACKAGE__){warn $_; $_++}}'
main at -e line 1.
maio at -e line 1.
maip at -e line 1.
maiq at -e line 1.
mair at -e line 1.
mais at -e line 1.
mait at -e line 1.
maiu at -e line 1.
maiv at -e line 1.
maiw at -e line 1.

for my$p(__PACKAGE__){$p++for+1..55664
,;!print"Just another \u$p hacker,\n"}
[perl #119043] Allow utf8 up/downgrade on ro COWs

Commit 1913067 allowed COW constants to be read-only.  This broke
Glib, so I reverted it with ba36554e02.  That caused this bug to reë-
merge (I hadn’t realised that I had fixed it in 1913067):

perl -e 'for(1..10){for(__PACKAGE__){warn $_; $_++}}'
main at -e line 1.
maio at -e line 1.
maip at -e line 1.
maiq at -e line 1.
mair at -e line 1.

so I reverted the revert two commits ago.

Glib was triggering a read-only error because it called
sv_utf8_upgrade on a read-only COW scalar, and sv_utf8_upgrade does
sv_force_normal on COWs to de-COW them.  sv_force_normal croaks on
read-only scalars.

The real problem here is that sv_force_normal means ‘I am going to
modify this scalar’, yet sv_utf8_upgrade conceptually does not modify
the scalar, but only changes the internal representation.

Having to call sv_force_normal to get the *side effect* of de-COWing
without triggering the various other things it does is no good.

What we need is a separate sv_uncow function that sv_force_normal
uses.  This commit introduces such a function.