Allow stash elems to be array refs

These turn into CVs that return the contents of the array.  Future
commits will make constant.pm use these and also make them inlinable.

Even without inlining, these subs are faster, because they are XSUBs:

$ time ./perl -Ilib -e 'my @a=1..1000000; sub foo { @a } () = foo for 1..10'

real	0m3.725s
user	0m3.407s
sys	0m0.227s
$ time ./perl -Ilib -e 'my @a=1..1000000; BEGIN { $::{foo} = \@a } () = foo for 1..10'

real	0m2.153s
user	0m1.949s
sys	0m0.121s
constant.pm: Make list constants read-only

Here we take advantage of the array-ref-stash-elem mechanism added in
the previous commit, which causes the actual elements of the stored
array to be pushed on to the stack.
Inline list constants

These are inlined the same way as 1..5.  We have two ops:

  rv2av
    |
    `-- const

The const op returns an AV, which is stored in the op tree, and then
rv2av flattens it.
Update constant.pm to reflect list inlinement

I also removed ‘some symbols may be redefined without generating a
warning’.  I know not to what it refers.  It has been there as long as
constant.pm has.  If a constant is clobbered by another constant with
the same value, there is no warning, as it is harmless.  That may be
to what it refers, but we don’t need a caveat for that.
Tweak optree_constants.t for inlined list consts
Make sub(){42} return a mutable value

But only make it do so in lvalue context.  This will be just as fast
in true rvalue context.  In either case, it is still inlined.

This makes sub () { 42 } and sub () { return 42 } do the same thing.

It also means that sub () { '-'x75 } reverts back to returning a muta-
ble value, the way it did in 5.16.  From now on, tweaks to constant
folding will no longer affect the mutability of the return value of a
nullary function.

‘use constant’ is unaffected.  It still returns read-only values.

This was brought up in ticket #109744.