* cppfiles.c: Include splay-tree.h, not hashtab.h.
(redundant_include_p, make_IHASH, hash_IHASH, eq_IHASH): Delete.
(destroy_include_file_node): New.
(_cpp_init_include_hash): Rename _cpp_init_include_table.
Create a splay tree, not a hash table.
(open_include_file): Look up the path in the include table,
do the multiple include optimization here, etc.
(cpp_included): Walk the path.
(find_include_file): Just walk the path calling
open_include_file, or call it directly for an absolute path.
(_cpp_fake_ihash): Rename _cpp_fake_include and update for new
scheme.
(read_include_file): Update for new scheme. Don't close the
file unless reading fails.
(_cpp_execute_include, cpp_read_file): Tweak for new scheme.