Add `FuzzXxx` tests to cipher algorithms

[raklaptudirm]

Description
Use the new go1.18 FuzzXxx tests to test cipher algorithm encryption and decryption.
The tests should accept fuzzed strings, encrypt then decrypt them, and check for equality with the original. This can help catch more elusive bugs in the code.

[HectorMalot]

I looked at this for a bit, but have a few questions for the fuzzing. As I'm writing the tests, I'm finding some edge cases and wanted to ask first if/how you want algorithms to change

1. For rot13/caesar encryption: the current algorithms only expect a-zA-Z as input, but don't validate this. This can be either be solved on the fuzzer (only allowing valid inputs), or on the algorithm (providing a helpful error is an input is outside of the expected range. Any preference or other ideas?

2. Similarly, RSA encrypt/decrypt as currently tested fails on certain inputs. Specifically, in the test p and q are set to 61 and 53, leading to a modulus of 3233. e.g. try encrypting and then decrypting string("\x82") with these exponents. This fails because the expected output is int64(65533) but the actual output is int64(873), which is equal to 65533%3233. Notably, every input above rune(3232) will fail because the modulus is not large enough.

   • Option 1: Encrypt byte by byte (the current string-rune conversion can sometimes return a different output than expected)
   • Option 2: invalidate modulus smaller than MaxInt64
   • Option 3: invalidate inputs with runes larger than the provided modulus
   • Option 4: leave as is, update test to larger modulus (I'll share a PR that does this a bit later)

Any preference / ideas on the above?

[siriak]

1. Validate input in the algorithm
2. Option 1 or 2 whichever is simpler