General Improvements (#47)

* Introduce `Result` shorthand, re-export and rename `InternalError`

* Re-export some public API relevant types

* Move `deserialize`

* Remove branch in `i2osp`

* Make `serialize` and `serialize_owned` methods

* Update p256

src/error.rs

@@ -0,0 +1,42 @@
+// Copyright (c) Facebook, Inc. and its affiliates.
+//
+// This source code is licensed under both the MIT license found in the
+// LICENSE-MIT file in the root directory of this source tree and the Apache
+// License, Version 2.0 found in the LICENSE-APACHE file in the root directory
+// of this source tree.
+
+//! Errors which are produced during an execution of the protocol
+
+use displaydoc::Display;
+
+/// [`Result`](core::result::Result) shorthand that uses [`Error`].
+pub type Result<T> = core::result::Result<T, Error>;
+
+/// Represents an error in the manipulation of internal cryptographic data
+#[derive(Clone, Copy, Debug, Display, Eq, Hash, Ord, PartialEq, PartialOrd)]
+pub enum Error {
+    /// Could not parse byte sequence for key
+    InvalidByteSequence,
+    /// Could not deserialize element, or deserialized to the identity element
+    PointError,
+    /// Computing the hash-to-curve function failed
+    HashToCurveError,
+    /// Failure to serialize or deserialize bytes
+    SerializationError,
+    /// Use of incompatible modes (base vs. verifiable)
+    IncompatibleModeError,
+    /**
+     * Internal error thrown when different-lengthed slices are supplied
+     * to the compute_composites() function.
+     */
+    MismatchedLengthsForCompositeInputs,
+    /// In verifiable mode, occurs when the proof failed to verify
+    ProofVerificationError,
+    /// Encountered insufficient bytes when attempting to deserialize
+    SizeError,
+    /// Encountered a zero scalar
+    ZeroScalarError,
+}
+
+#[cfg(feature = "std")]
+impl std::error::Error for Error {}