Use rand-std in key rustdoc examples

Seems there is a bug in cargo, the tests in `key.rs` run successfully
but AFAICT they should fail. Here is an example, running `cargo test
--features=rand` should make this test fail but it doesn't?
```
/// Secret 256-bit key used as `x` in an ECDSA signature.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// # #[cfg(all(feature = "rand", any(feature =  "alloc", feature = "std")))] {
/// use secp256k1::{rand, Secp256k1, SecretKey};
///
/// let secp = Secp256k1::new();
/// let secret_key = SecretKey::new(&mut rand::thread_rng());
/// # }
/// ```

Anywho, use the correct feature gate: `rand-std`.

Author: tcharding

src/key.rs

@@ -35,7 +35,7 @@ use ffi::{self, CPtr};
 /// Basic usage:
 ///
 /// ```
-/// # #[cfg(all(feature = "rand", any(feature =  "alloc", feature = "std")))] {
+/// # #[cfg(all(feature = "rand-std", any(feature =  "alloc", feature = "std")))] {
 /// use secp256k1::{rand, Secp256k1, SecretKey};
 ///
 /// let secp = Secp256k1::new();
@@ -129,7 +129,7 @@ impl SecretKey {
     /// # Examples
     ///
     /// ```
-    /// # #[cfg(feature="rand")] {
+    /// # #[cfg(all(feature = "rand-std", any(feature =  "alloc", feature = "std")))] {
     /// use secp256k1::{rand, SecretKey};
     /// let secret_key = SecretKey::new(&mut rand::thread_rng());
     /// # }
@@ -185,7 +185,7 @@ impl SecretKey {
     /// # Examples
     ///
     /// ```
-    /// # #[cfg(all(feature = "rand", any(feature =  "alloc", feature = "std")))] {
+    /// # #[cfg(all(feature = "rand-std", any(feature =  "alloc", feature = "std")))] {
     /// use secp256k1::{rand, Secp256k1, SecretKey, KeyPair};
     ///
     /// let secp = Secp256k1::new();
@@ -329,7 +329,7 @@ impl PublicKey {
     /// # Examples
     ///
     /// ```
-    /// # #[cfg(all(feature = "rand", any(feature =  "alloc", feature = "std")))] {
+    /// # #[cfg(all(feature = "rand-std", any(feature =  "alloc", feature = "std")))] {
     /// use secp256k1::{rand, Secp256k1, SecretKey, PublicKey};
     ///
     /// let secp = Secp256k1::new();
@@ -377,7 +377,7 @@ impl PublicKey {
     /// # Examples
     ///
     /// ```
-    /// # #[cfg(all(feature = "rand", any(feature =  "alloc", feature = "std")))] {
+    /// # #[cfg(all(feature = "rand-std", any(feature =  "alloc", feature = "std")))] {
     /// use secp256k1::{rand, Secp256k1, PublicKey, KeyPair};
     ///
     /// let secp = Secp256k1::new();
@@ -508,7 +508,7 @@ impl PublicKey {
     /// # Examples
     ///
     /// ```
-    /// # #[cfg(all(feature = "rand", any(feature =  "alloc", feature = "std")))] {
+    /// # #[cfg(all(feature = "rand-std", any(feature =  "alloc", feature = "std")))] {
     /// use secp256k1::{rand, Secp256k1};
     ///
     /// let secp = Secp256k1::new();
@@ -534,7 +534,7 @@ impl PublicKey {
     /// # Examples
     ///
     /// ```
-    /// # #[cfg(all(feature = "rand", any(feature =  "alloc", feature = "std")))] {
+    /// # #[cfg(all(feature = "rand-std", any(feature =  "alloc", feature = "std")))] {
     /// use secp256k1::{rand, Secp256k1, PublicKey};
     ///
     /// let secp = Secp256k1::new();
@@ -650,7 +650,7 @@ impl Ord for PublicKey {
 /// Basic usage:
 ///
 /// ```
-/// # #[cfg(all(feature = "rand", any(feature =  "alloc", feature = "std")))] {
+/// # #[cfg(all(feature = "rand-std", any(feature =  "alloc", feature = "std")))] {
 /// use secp256k1::{rand, KeyPair, Secp256k1};
 ///
 /// let secp = Secp256k1::new();
@@ -744,7 +744,7 @@ impl KeyPair {
     /// # Examples
     ///
     /// ```
-    /// # #[cfg(all(feature = "rand", any(feature =  "alloc", feature = "std")))] {
+    /// # #[cfg(all(feature = "rand-std", any(feature =  "alloc", feature = "std")))] {
     /// use secp256k1::{rand, Secp256k1, SecretKey, KeyPair};
     ///
     /// let secp = Secp256k1::new();
@@ -790,7 +790,7 @@ impl KeyPair {
     /// # Examples
     ///
     /// ```
-    /// # #[cfg(all(feature = "rand", any(feature =  "alloc", feature = "std")))] {
+    /// # #[cfg(all(feature = "rand-std", any(feature =  "alloc", feature = "std")))] {
     /// use secp256k1::{Secp256k1, KeyPair};
     /// use secp256k1::rand::{RngCore, thread_rng};
     ///
@@ -914,7 +914,7 @@ impl<'de> ::serde::Deserialize<'de> for KeyPair {
 /// Basic usage:
 ///
 /// ```
-/// # #[cfg(all(feature = "rand", any(feature =  "alloc", feature = "std")))] {
+/// # #[cfg(all(feature = "rand-std", any(feature =  "alloc", feature = "std")))] {
 /// use secp256k1::{rand, Secp256k1, KeyPair, XOnlyPublicKey};
 ///
 /// let secp = Secp256k1::new();
@@ -1042,7 +1042,7 @@ impl XOnlyPublicKey {
     /// # Examples
     ///
     /// ```
-    /// # #[cfg(all(feature = "rand", any(feature =  "alloc", feature = "std")))] {
+    /// # #[cfg(all(feature = "rand-std", any(feature =  "alloc", feature = "std")))] {
     /// use secp256k1::{Secp256k1, KeyPair};
     /// use secp256k1::rand::{RngCore, thread_rng};
     ///
@@ -1107,7 +1107,7 @@ impl XOnlyPublicKey {
     /// # Examples
     ///
     /// ```
-    /// # #[cfg(all(feature = "rand", any(feature = "alloc", feature = "std")))] {
+    /// # #[cfg(all(feature = "rand-std", any(feature = "alloc", feature = "std")))] {
     /// use secp256k1::{Secp256k1, KeyPair};
     /// use secp256k1::rand::{thread_rng, RngCore};
     ///