std: Remove public bool,tuple,unit modules

This commit modifies rustdoc to not require these empty modules to be public in
the standard library. The modules still remain as a location to attach
documentation to, but the modules themselves are now private (don't have to
commit to an API). The documentation for the standard library now shows all of
the primitive types on the main index page.

src/libstd/lib.rs

@@ -137,7 +137,6 @@ extern crate rustrt;
 // NB: These reexports are in the order they should be listed in rustdoc
 
 pub use core::any;
-pub use core::bool;
 pub use core::borrow;
 pub use core::cell;
 pub use core::clone;
@@ -152,10 +151,6 @@ pub use core::mem;
 pub use core::ptr;
 pub use core::raw;
 pub use core::simd;
-pub use core::tuple;
-// FIXME #15320: primitive documentation needs top-level modules, this
-// should be `std::tuple::unit`.
-pub use core::unit;
 pub use core::result;
 pub use core::option;
 
@@ -246,6 +241,12 @@ pub mod comm;
 pub mod rt;
 mod failure;
 
+// Documentation for primitive types
+
+mod bool;
+mod unit;
+mod tuple;
+
 // A curious inner-module that's not exported that contains the binding
 // 'std' so that macro-expanded references to std::error and such
 // can be resolved within libstd.