rage-mount: Add support for mounting an encrypted file

[str4d]

This behaves similarly to mount --bind source_file target_file, except
that source_file is transparently decrypted, and target_file is
created as a symbolic link instead of a bind-mount.

Part of #188.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 87 lines in your changes missing coverage. Please review.

Project coverage is 33.72%. Comparing base (9c56470) to head (c0509ca).
Report is 571 commits behind head on main.

Files	Patch %	Lines
rage/src/bin/rage-mount/file.rs	0.00%	83 Missing ⚠️
rage/src/bin/rage-mount/main.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #190      +/-   ##
==========================================
- Coverage   34.90%   33.72%   -1.18%     
==========================================
  Files          33       34       +1     
  Lines        2716     2802      +86     
==========================================
- Hits          948      945       -3     
- Misses       1768     1857      +89     

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