core: Rename container mod to collections. Closes #12543

Also renames the `Container` trait to `Collection`.

[breaking-change]

src/libstd/os.rs

@@ -30,7 +30,7 @@
 #![allow(non_snake_case_functions)]
 
 use clone::Clone;
-use container::Container;
+use collections::Collection;
 use fmt;
 use iter::Iterator;
 use libc::{c_void, c_int};