fix: improve edge case detection in HashMap and BTreeMap codec decoding

[tomleavy]

Description of changes:

This resolves two issues

1. HashMap and BTreeMap MlsDecode implementation allows for duplicate keys to be discarded, which should throw an error instead
2. HashMap and BTreeMap MlsDecode fails if presented with a specially crafted payload + zero length structure target for decode

Testing:

This file lacked proper tests, which have now been added

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT license.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.82%. Comparing base (8b8b521) to head (15e0363).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #316      +/-   ##
==========================================
+ Coverage   88.70%   88.82%   +0.12%     
==========================================
  Files         182      182              
  Lines       29009    29174     +165     
==========================================
+ Hits        25732    25914     +182     
+ Misses       3277     3260      -17     

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[stefunctional]

The implementation for Vec<T> suffers from the same problem when T is zero-sized, leading to an infinite loop for a malicious payload pretending to have a non-empty Vec.

[stefunctional]

It looks like git add mls-rs-codec/src/vec.rs might have been forgotten before the second commit.