#5732 Use unique_ptr in layout code

[Dan Eble]

Use std::unique_ptr instead of manually managed raw pointers.

I suggest beginning the review in Accidental_placement_entry. The
algorithm in stagger_apes () took me the most time to come to term
with. With unique_ptr, it's much easier to be confident that there
isn't a memory leak there.

unique_ptr deletes its object when it goes out of scope. Transferring
ownership of the object to another instance of unique_ptr is done with
std::move(). A unique_ptr can not be copied, but one can get an alias
(a raw pointer) from it with p.get().

Using unique_ptr makes the flower junk_pointers() function
unnecessary. I can't remove junk_pointers() yet because MIDI code
still uses it, and I don't want to wade into that high water now.

I'm aware that some lines are longer than 80 characters. I plan to
try clang-format after Han-Wen's config file makes it through the
countdown.

https://codereview.appspot.com/573500043

[Anonymous]

Passes make, make check and a full make doc

[Anonymous]

Patch on countdown for Feb 6th.

[Anonymous]

Patch counted down - please push.

[Dan Eble]

commit 011a4593fcc6450e9cb1fc0fff9bc3e5b1395a9b
Author: Dan Eble <nine.fierce.ballads@gmail.com>
Date:   Sun Feb 2 21:42:06 2020 -0500

    Issue 5732/4: Use unique_ptr for Slur_configuration

commit e42552f5fac039a73018d2fcb4a11f0fc1a7b9f7
Author: Dan Eble <nine.fierce.ballads@gmail.com>
Date:   Sun Feb 2 21:50:50 2020 -0500

    Issue 5732/3: Use unique_ptr in Bracket_nesting_group

commit 92590f43038b346a8fb6b1901cdf92f2e193a327
Author: Dan Eble <nine.fierce.ballads@gmail.com>
Date:   Sun Feb 2 22:05:42 2020 -0500

    Issue 5732/2: Use unique_ptr for Beam_configuration

commit d317c9675c6ff7acc1c5b399754e95a3103f8b2e
Author: Dan Eble <nine.fierce.ballads@gmail.com>
Date:   Mon Feb 3 11:19:05 2020 -0500

    Issue 5732/1: Use unique_ptr for Accidental_placement_entry