rustdoc: Support for "array" primitive

Impls on `clean::Type::FixedVector` are now collected in the array
primitive page instead of the slice primitive page.

Also add a primitive docs for arrays to `std`.

src/libstd/array.rs

@@ -0,0 +1,13 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! The fixed-size array type (`[T; n]`).
+
+#![doc(primitive = "array")]