build: make tests depend on the library
System: remove printfs from `file_info`
System: add function `remove`
sys_test: test System.remove
build: fix test dependencies
System: add function `list_directory`
System: add function make_symlink
sys_test: use System.make_symlink (in test_remove)
System: add function copy
sys_test: test function System.copy
System: add function `file_tree`
sys_test: use System.file_tree in `test_copy`
System: add functions move_in_same_device and move
sys_test: test System.move (c.f. note)

Note: to test the `move` function in the `must_copy` case, one has to
launch the test from another partition/device than the one containing
"/tmp".
build: fix build of the API documentation