Remove ONE_KEY

The `ONE_KEY` is only used in two rustdoc examples, as such it
unnecessarily pollutes the crate root namespace. We can use
`SecretKey::from_str()` with no loss of clarity and remove the
`ONE_KEY`.

While we are touching the import statements in `secret.rs` elect to
remove the hide (use of `#`) for import statements relating to this
library. Doing so gives devs all the information they need in one place
if they are using the examples to copy code. It is also in line with the
rest of the codebase.

src/key.rs

@@ -74,9 +74,6 @@ impl str::FromStr for SecretKey {
     }
 }
 
-/// The number 1 encoded as a secret key.
-pub const ONE_KEY: SecretKey = SecretKey(constants::ONE);
-
 /// A Secp256k1 public key, used for verification of signatures.
 ///
 /// # Serde support

src/secret.rs

@@ -118,7 +118,9 @@ impl SecretKey {
     ///
     /// ```
     /// # #[cfg(feature = "std")] {
-    /// let key = secp256k1::ONE_KEY;
+    /// # use std::str::FromStr;
+    /// use secp256k1::SecretKey;
+    /// let key = SecretKey::from_str("0000000000000000000000000000000000000000000000000000000000000001").unwrap();
     ///
     /// // Normal debug hides value (`Display` is not implemented for `SecretKey`).
     /// // E.g., `format!("{:?}", key)` prints "SecretKey(#2518682f7819fb2d)".
@@ -152,12 +154,11 @@ impl KeyPair {
     ///
     /// ```
     /// # #[cfg(feature = "std")] {
-    /// use secp256k1::ONE_KEY;
-    /// use secp256k1::KeyPair;
-    /// use secp256k1::Secp256k1;
+    /// # use std::str::FromStr;
+    /// use secp256k1::{KeyPair, Secp256k1, SecretKey};
     ///
     /// let secp = Secp256k1::new();
-    /// let key = ONE_KEY;
+    /// let key = SecretKey::from_str("0000000000000000000000000000000000000000000000000000000000000001").unwrap();
     /// let key = KeyPair::from_secret_key(&secp, &key);
     /// // Here we explicitly display the secret value:
     /// assert_eq!(
@@ -190,7 +191,7 @@ impl SharedSecret {
     /// # #[cfg(not(fuzzing))]
     /// # #[cfg(feature = "std")] {
     /// # use std::str::FromStr;
-    /// # use secp256k1::{SecretKey, PublicKey};
+    /// use secp256k1::{SecretKey, PublicKey};
     /// use secp256k1::ecdh::SharedSecret;
     ///
     /// # let pk = PublicKey::from_slice(&[3, 23, 183, 225, 206, 31, 159, 148, 195, 42, 67, 115, 146, 41, 248, 140, 11, 3, 51, 41, 111, 180, 110, 143, 114, 134, 88, 73, 198, 174, 52, 184, 78]).expect("hard coded slice should parse correctly");