Add rand-std feature

Currently we enable "secp256k1/rand-std" in the "rand" feature, this is
incorrect because it means "rand" implies "std" which it does not.

Add a "rand-std" feature that turns on "seck256k1/rand-std" and make the
"rand" feature turn on "seck256k1/rand".

bitcoin/Cargo.toml

@@ -15,7 +15,8 @@ edition = "2018"
 # Please don't forget to add relevant features to docs.rs below
 [features]
 default = [ "std", "secp-recovery" ]
-rand = ["secp256k1/rand-std"]
+rand-std = ["secp256k1/rand-std"]
+rand = ["secp256k1/rand"]
 serde = ["actual-serde", "bitcoin_hashes/serde", "secp256k1/serde"]
 secp-lowmemory = ["secp256k1/lowmemory"]
 secp-recovery = ["secp256k1/recovery"]

bitcoin/src/address.rs

@@ -8,24 +8,23 @@
 //! # Example: creating a new address from a randomly-generated key pair
 //!
 //! ```rust
-//! use bitcoin::network::constants::Network;
-//! use bitcoin::address::Address;
-//! use bitcoin::PublicKey;
-//! use bitcoin::secp256k1::Secp256k1;
-//! use bitcoin::secp256k1::rand::thread_rng;
+//! # #[cfg(feature = "rand-std")] {
+//! use bitcoin::{Address, PublicKey, Network};
+//! use bitcoin::secp256k1::{rand, Secp256k1};
 //!
 //! // Generate random key pair.
 //! let s = Secp256k1::new();
-//! let public_key = PublicKey::new(s.generate_keypair(&mut thread_rng()).1);
+//! let public_key = PublicKey::new(s.generate_keypair(&mut rand::thread_rng()).1);
 //!
 //! // Generate pay-to-pubkey-hash address.
 //! let address = Address::p2pkh(&public_key, Network::Bitcoin);
+//! # }
 //! ```
 //!
-//! # Note: creating a new address requires the rand feature flag
+//! # Note: creating a new address requires the rand-std feature flag
 //!
 //! ```toml
-//! bitcoin = { version = "...", features = ["rand"] }
+//! bitcoin = { version = "...", features = ["rand-std"] }
 //! ```
 
 use core::convert::TryFrom;

bitcoin/src/psbt/mod.rs

@@ -887,7 +887,7 @@ mod tests {
     use crate::hash_types::Txid;
 
     use secp256k1::{Secp256k1, self};
-    #[cfg(feature = "rand")]
+    #[cfg(feature = "rand-std")]
     use secp256k1::{All, SecretKey};
 
     use crate::blockdata::script::ScriptBuf;
@@ -1685,7 +1685,7 @@ mod tests {
         assert_eq!(psbt1, psbt2);
     }
 
-    #[cfg(feature = "rand")]
+    #[cfg(feature = "rand-std")]
     fn gen_keys() -> (PrivateKey, PublicKey, Secp256k1<All>) {
         use secp256k1::rand::thread_rng;
 
@@ -1699,7 +1699,7 @@ mod tests {
     }
 
     #[test]
-    #[cfg(feature = "rand")]
+    #[cfg(feature = "rand-std")]
     fn get_key_btree_map() {
         let (priv_key, pk, secp) = gen_keys();
 
@@ -1811,7 +1811,7 @@ mod tests {
     }
 
     #[test]
-    #[cfg(feature = "rand")]
+    #[cfg(feature = "rand-std")]
     fn sign_psbt() {
         use crate::WPubkeyHash;
         use crate::bip32::{Fingerprint, DerivationPath};