redo: use find for true recursive source file searching
lib: remove jgfs2.h header inclusion in lib/tree.h
lib/tree: massively reorganize into smaller code files
lib: add note about higher-level extent allocation function
redo: add -flto
lib/tree: reorganize node code by function
lib/tree/tree: use finer locking in tree_insert
lib/tree/tree: partially implement tree_make_space
lib/tree/tree: continue work on insert/remove/split/merge
lib/tree/node: implement node_append_multiple