Allow the user to specify functions for allocating memory for splay tree roots and nodes.

include:
Allow the user to specify functions for allocating memory for
splay tree roots and nodes.
* splay-tree.h (splay_tree_allocate_fn, splay_tree_deallocate_fn):
New types.
(splay_tree): New fields: `allocate', `deallocate', and
`allocate_data'.
(splay_tree_new_with_allocator): New function declaration.

libiberty:
* splay-tree.c (splay_tree_xmalloc_allocate,
splay_tree_xmalloc_deallocate): New functions.
(splay_tree_new): Call splay_tree_new_with_allocator, passing the
above functions and a dummy data pointer.
(splay_tree_new_with_allocator): New function.
(splay_tree_delete_helper, splay_tree_delete, splay_tree_insert,
splay_tree_remove): Use the splay tree's allocation and
deallocation functions.

From-SVN: r49968