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.

TODO.md

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+# Roadmap 1.0
+## Summary
+Make the program real-world usable and stable.
+
+## Required changes
+* password hashing
+  * configurable algorithm (sane default)
+  * configurable nr of rounds (sand default)
+  * a way to enter the password securely in a prompt while still being able to handle `stdin` data
+* add usage examples to README.md
+* unit tests
+* understand `encrypt_next_in_place()`s first argument better
+  * current understanding:
+    * associated data is used for parts of the data that cannot be
+      encrypted but should also be integrity protected by the authentication tag
+    * since there are no parts that cannot be encrypted in the context of `fcry` it is correct
+      to pass an empty slice to the first argument of `encrypt_next_in_place()`
+
+# Roadmap 2.0
+* parallel processing: use all available (or configurable) CPU cores
+
+# Roadmap later or never
+* split into `lib` and `bin`
+* other AEAD algorithms