Add no_std support (#7)

* Add no_std support

* Don't depend on dependencies' std features in std

Cargo.toml

@@ -13,11 +13,13 @@ keywords = ["encryption", "xts"]
 exclude = ["test_files"]
 
 [features]
+default = ["std"]
+std = []
 openssl_tests = ["openssl"]
 
 [dependencies]
 cipher = "0.4"
-byteorder = "1"
+byteorder = { version = "1", default-features = false }
 # Is actually a dev-dependency enabled only for/by openssl_tests, but dev-dependencies can't be
 # optional
 openssl = { version = "0.10", optional = true }

src/lib.rs

@@ -1,3 +1,5 @@
+#![cfg_attr(not(feature = "std"), no_std)]
+
 /*!
 [XTS block mode](https://en.wikipedia.org/wiki/Disk_encryption_theory#XEX-based_tweaked-codebook_mode_with_ciphertext_stealing_(XTS)) implementation in Rust.
 
@@ -122,8 +124,8 @@ xts.decrypt_area(&mut buffer[0x400..0xC00], 0x200, 2, get_nintendo_tweak);
 ```
 */
 
-use std::convert::TryFrom;
-use std::convert::TryInto;
+use core::convert::TryFrom;
+use core::convert::TryInto;
 
 use byteorder::{ByteOrder, LittleEndian};
 use cipher::generic_array::typenum::Unsigned;