fcry - [f]ile[cry]pt - initial commit (alpha 0.9.0)

A file en-/decryption tool for easy use.

Currently `fcry` uses `ChaCha20Poly1305` ([RFC 8439](https://datatracker.ietf.org/doc/html/rfc8439)) as [AEAD](https://en.wikipedia.org/wiki/Authenticated_encryption) cipher provided by the [chacha20poly1305](https://docs.rs/chacha20poly1305/latest/chacha20poly1305/) crate.

src/error.rs

@@ -0,0 +1,28 @@
+// SPDX-License-Identifier: GPL-3.0-only
+use chacha20poly1305::aead;
+use std::io;
+
+#[derive(Debug)]
+pub enum FcryError {
+    Io(io::Error),
+    Crypto(aead::Error),
+    Rng(rand::Error),
+}
+
+impl From<io::Error> for FcryError {
+    fn from(e: io::Error) -> Self {
+        FcryError::Io(e)
+    }
+}
+
+impl From<aead::Error> for FcryError {
+    fn from(e: aead::Error) -> Self {
+        FcryError::Crypto(e)
+    }
+}
+
+impl From<rand::Error> for FcryError {
+    fn from(e: rand::Error) -> Self {
+        FcryError::Rng(e)
+    }
+}