core: Rename container mod to collections. Closes #12543

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

[breaking-change]

src/libstd/ascii.rs

@@ -10,7 +10,7 @@
 
 //! Operations on ASCII strings and characters
 
-use container::Container;
+use collections::Collection;
 use fmt;
 use iter::Iterator;
 use mem;