age: Add streamlined APIs for encryption and decryption

[str4d]

Closes #333.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 51.08%. Comparing base (9af4790) to head (195b86b).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
age/src/simple.rs	87.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #525      +/-   ##
==========================================
+ Coverage   50.87%   51.08%   +0.21%     
==========================================
  Files          42       43       +1     
  Lines        4136     4160      +24     
==========================================
+ Hits         2104     2125      +21     
- Misses       2032     2035       +3     

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