Consolidate crypto functionality in ethcore-crypto. (#8432)

* Consolidate crypto functionality in `ethcore-crypto`.

- Move `ecdh`/`ecies` modules to `ethkey`.
- Refactor `ethcore-crypto` to use file per module.
- Replace `subtle` with `ethcore_crypto::is_equal`.
- Add `aes_gcm` module to `ethcore-crypto`.

* Rename `aes::{encrypt,decrypt,decrypt_cbc}` ...

... to `aes::{encrypt_128_ctr,decrypt_128_ctr,decrypt_128_cbc}`.

ethcore/crypto/src/error.rs

@@ -0,0 +1,83 @@
+// Copyright 2015-2017 Parity Technologies (UK) Ltd.
+// This file is part of Parity.
+
+// Parity is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity.  If not, see <http://www.gnu.org/licenses/>.
+
+use rcrypto;
+use ring;
+
+quick_error! {
+	#[derive(Debug)]
+	pub enum Error {
+		Scrypt(e: ScryptError) {
+			cause(e)
+			from()
+		}
+		Symm(e: SymmError) {
+			cause(e)
+			from()
+		}
+	}
+}
+
+quick_error! {
+	#[derive(Debug)]
+	pub enum ScryptError {
+		// log(N) < r / 16
+		InvalidN {
+			display("Invalid N argument of the scrypt encryption")
+		}
+		// p <= (2^31-1 * 32)/(128 * r)
+		InvalidP {
+			display("Invalid p argument of the scrypt encryption")
+		}
+	}
+}
+
+quick_error! {
+	#[derive(Debug)]
+	pub enum SymmError wraps PrivSymmErr {
+		RustCrypto(e: rcrypto::symmetriccipher::SymmetricCipherError) {
+			display("symmetric crypto error")
+			from()
+		}
+		Ring(e: ring::error::Unspecified) {
+			display("symmetric crypto error")
+			cause(e)
+			from()
+		}
+		Offset(x: usize) {
+			display("offset {} greater than slice length", x)
+		}
+	}
+}
+
+impl SymmError {
+	pub(crate) fn offset_error(x: usize) -> SymmError {
+		SymmError(PrivSymmErr::Offset(x))
+	}
+}
+
+impl From<ring::error::Unspecified> for SymmError {
+	fn from(e: ring::error::Unspecified) -> SymmError {
+		SymmError(PrivSymmErr::Ring(e))
+	}
+}
+
+impl From<rcrypto::symmetriccipher::SymmetricCipherError> for SymmError {
+	fn from(e: rcrypto::symmetriccipher::SymmetricCipherError) -> SymmError {
+		SymmError(PrivSymmErr::RustCrypto(e))
+	}
+}
+