core: Rename container mod to collections. Closes #12543

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

[breaking-change]

src/libstd/io/mod.rs

@@ -214,7 +214,7 @@ responding to errors that may occur while attempting to read the numbers.
 #![deny(unused_must_use)]
 
 use char::Char;
-use container::Container;
+use collections::Collection;
 use fmt;
 use int;
 use iter::Iterator;