Add an alternative splay tree implementation

Add an alternative splay tree implementation

We already have two splay tree implementations: the old C one in
libiberty and a templated reimplementation of it in typed-splay-tree.h.
However, they have some drawbacks:

- They hard-code the assumption that nodes should have both a key and
  a value, which isn't always true.

- They use the two-phase method of lookup, and so nodes need to store
  a temporary back pointer.  We can avoid that overhead by using the
  top-down method (as e.g. the bitmap tree code already does).

- The tree node has to own the key and the value.  For some use cases
  it's more convenient to embed the tree links in the value instead.

Also, a later patch wants to use splay trees to represent an
adaptive total order: the splay tree itself records whether node N1
is less than node N2, and (in the worst case) comparing nodes is
a splay operation.

This patch therefore adds an alternative implementation.  The main
features are:

- Nodes can optionally point back to their parents.

- An Accessors class abstracts accessing child nodes and (where
  applicable) parent nodes, so that the information can be embedded
  in larger data structures.

- There is no fixed comparison function at the class level.  Instead,
  individual functions that do comparisons take a comparison function
  argument.

- There are two styles of comparison function, optimised for different
  use cases.  (See the comments in the patch for details.)

- It's possible to do some operations directly on a given node,
  without knowing whether it's the root.  This includes the comparison
  use case described above.

This of course has its own set of drawbacks.  It's really providing
splay utility functions rather than a true ADT, and so is more low-level
than the existing routines.  It's mostly geared for cases in which the
client code wants to participate in the splay operations to some extent.

gcc/
	* Makefile.in (OBJS): Add splay-tree-utils.o.
	* system.h: Include <array> when INCLUDE_ARRAY is defined.
	* selftest.h (splay_tree_cc_tests): Declare.
	* selftest-run-tests.c (selftest::run_tests): Run splay_tree_cc_tests.
	* splay-tree-utils.h: New file.
	* splay-tree-utils.tcc: Likewise.
	* splay-tree-utils.cc: Likewise.