Squash of PR#42 from @xoloki (dalek-cryptography#42)

first pass nostd, turn off default features and add std/alloc features

Update Cargo.toml

Update Cargo.toml

use core ops when in alloc mode

chain features

remove std::ops entirely since we always have core anyway

remove hex and debug transcript to get rid of std errors

alloc default

set no_std config attribute

remove core crate extern

import core crate before use

don't add extern crate core; use global namespace when referring to core

include core explicitly if we're in std mode

re-enable debug-transcript feature

re-enable debug-transcript feature

fmt fixes

Cargo.toml

@@ -18,11 +18,11 @@ exclude = [".travis.yml", ".gitignore"]
 features = ["nightly"]
 
 [dependencies]
-keccak = "0.1.0"
-byteorder = "1.2.4"
-clear_on_drop = "0.2.3"
-rand_core = "0.3"
-hex = {version = "0.3", optional = true}
+keccak = { version = "0.1.0", default-features = false }
+byteorder = { version = "1.2.4", default-features = false }
+clear_on_drop = { version = "0.2.3", default-features = false }
+rand_core = { version = "0.3", default-features = false }
+hex = {version = "0.3", default-features = false, optional = true}
 
 [dev-dependencies]
 strobe-rs = "0.3"
@@ -31,5 +31,8 @@ rand_chacha = "0.1"
 rand = "0.6"
 
 [features]
+default = ["std"]
 nightly = ["clear_on_drop/nightly"]
 debug-transcript = ["hex"]
+std = ["rand_core/std", "byteorder/std"]
+alloc = ["rand_core/alloc"]

src/lib.rs

@@ -1,3 +1,5 @@
+#![cfg_attr(not(feature = "std"), no_std)]
+#![cfg_attr(feature = "alloc", feature(alloc))]
 #![cfg_attr(feature = "nightly", feature(external_doc))]
 #![cfg_attr(feature = "nightly", doc(include = "../README.md"))]
 #![doc(html_logo_url = "https://doc.dalek.rs/assets/dalek-logo-clear.png")]
@@ -16,10 +18,12 @@ please file an issue!
 
 extern crate byteorder;
 extern crate clear_on_drop;
-extern crate core;
 extern crate keccak;
 extern crate rand_core;
 
+#[cfg(feature = "std")]
+extern crate core;
+
 #[cfg(test)]
 extern crate curve25519_dalek;
 #[cfg(feature = "hex")]

src/strobe.rs

@@ -1,7 +1,7 @@
 //! Minimal implementation of (parts of) Strobe.
 
 use keccak;
-use std::ops::{Deref, DerefMut};
+use ::core::ops::{Deref, DerefMut};
 
 /// Strobe R value; security level 128 is hardcoded
 const STROBE_R: u8 = 166;