Make std/src/num mirror core/src/num

The float modules in `std` are currently top-level but for `core`, they
are nested within the `num` directory and referenced by `#[path = ...]`.
For consistency, adjust `std` to use the same structure as `core`.

Also change the `f16` and `f128` gates from outer attributes to inner
attributes like `core` has.

library/std/src/lib.rs

@@ -585,11 +585,13 @@ pub use alloc_crate::string;
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use alloc_crate::vec;
 
-#[unstable(feature = "f128", issue = "116909")]
+#[path = "num/f128.rs"]
 pub mod f128;
-#[unstable(feature = "f16", issue = "116909")]
+#[path = "num/f16.rs"]
 pub mod f16;
+#[path = "num/f32.rs"]
 pub mod f32;
+#[path = "num/f64.rs"]
 pub mod f64;
 
 #[macro_use]

library/std/src/num/f128.rs

@@ -4,6 +4,8 @@
 //!
 //! Mathematically significant numbers are provided in the `consts` sub-module.
 
+#![unstable(feature = "f128", issue = "116909")]
+
 #[unstable(feature = "f128", issue = "116909")]
 pub use core::f128::consts;
 

library/std/src/num/f16.rs

@@ -4,6 +4,8 @@
 //!
 //! Mathematically significant numbers are provided in the `consts` sub-module.
 
+#![unstable(feature = "f16", issue = "116909")]
+
 #[unstable(feature = "f16", issue = "116909")]
 pub use core::f16::consts;