Add #![feature(generic_const_exprs)] to doctests

Some of them were missing this feature annotation but used to compile
and pass for some inexplicable reason. Now this missing feature gate
causes an ICE (rust-lang/rust#133199). This
adds the feature gates in all the doctests, but does not yet address the
fact that this ICE also occurs in calling crates that don't have
`#![feature(generic_const_exprs)]` set. Maybe a workaround could be to
check for this feature explicitly and throw a helpful error message in
diman.

src/lib.rs

@@ -60,6 +60,7 @@
 //! ## Arithmetics and math
 //! Addition and subtraction of two quantities is allowed if the dimensions match:
 //! ```
+//! # #![feature(generic_const_exprs)]
 //! # use diman::si::dimensions::{Length};
 //! # use diman::si::units::{kilometers, meters};
 //! let l = 5.0 * meters + 10.0 * kilometers;
@@ -94,6 +95,7 @@
 //! ```
 //! Exponentiation and related operations are supported via `squared`, `cubed`, `powi`, `sqrt`, `cbrt`:
 //! ```
+//! # #![feature(generic_const_exprs)]
 //! # use diman::si::dimensions::{Length};
 //! # use diman::si::units::{meters, cubic_meters, square_meters};
 //! let length = 2.0f64 * meters;
@@ -139,6 +141,7 @@
 //!
 //! Conversion into the underlying storage type can be done using the `value_in` function:
 //! ```
+//! # #![feature(generic_const_exprs)]
 //! # use diman::si::units::{kilometers, meters};
 //! let length = 2.0f64 * kilometers;
 //! assert_eq!(format!("{} m", length.value_in(meters)), "2000 m");
@@ -181,6 +184,7 @@
 //! ```
 //! The combination of `value_unchecked` and `new_unchecked` comes in handy when using third party libraries that only takes the raw storage type as argument. As an example, suppose we have a function `foo` that takes a `Vec<f64>` and returns a `Vec<f64>`, and suppose it sorts the numbers or does some other unit safe operation. Then we could reasonably write:
 //! ```
+//! # #![feature(generic_const_exprs)]
 //! # use diman::si::dimensions::{Length};
 //! # use diman::si::units::{meters, kilometers};
 //! # fn foo(x: Vec<f64>) -> Vec<f64> {
@@ -312,6 +316,7 @@
 //! not needed too many times. Having to add a definition to the unit system for this case can be cumbersome.
 //! This is why the `Product` and `Quotient` types are provided:
 //! ```
+//! # #![feature(generic_const_exprs)]
 //! use diman::si::dimensions::{Length, Time};
 //! use diman::{Product, Quotient};
 //!