move Rust port to main branch

[vgough]

This contains a Rust port, which currently implements read-only mode, and some write support.
Both encfs and encfsctl are implemented, although in simplified form.

The C++ code has been moved to a legacy folder in order to focus on the Rust implementation.

This is a work in progress.

---

Note

Replaces the C++ implementation with a new primary Rust crate and modern CI.

• Adds Rust crate encfs with config loaders (V4/V5/V6 XML/binary), OpenSSL-based crypto (ssl), file encoder/decoder (crypto/file), and a functional encfsctl CLI (info, passwd, encode/decode, cat, ls, showKey, export), plus extensive tests
• Moves the old C++ code into legacy/ and adjusts its CMake paths/includes
• Introduces GitHub Actions workflow and Cirrus FreeBSD job; removes Travis and obsolete dev scripts; adds Taskfile.yml, .gitignore, and .gitattributes
• Updates licensing and docs (COPYING, INSTALL.md, README.md, ChangeLog) to reflect Rust-first direction and build instructions

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