UTF8-De-/Encoding for MatZ

[jnsiemer]

Description

This PR implements...

• [x] padding for UTF8-encoded strings to ensure that no problems happen along the way of decoding (including major progress for MatZq already)
• [x] from_utf8
• [x] to_utf

for MatZ.

Testing

• [x] I added basic working examples (possibly in doc-comment)
• [x] I triggered all possible errors in my test in every possible way
• [x] I included tests for all reasonable edge cases

Checklist:

• [x] I have performed a self-review of my own code
  • [x] The code provides good readability and maintainability s.t. it fulfills best practices like talking code, modularity, ...
  • [x] The chosen implementation is not more complex than it has to be
  • [x] My code should work as intended and no side effects occur (e.g. memory leaks)
  • [x] The doc comments fit our style guide