fix: store key-value pairs in the hashmap preserving the order of insertions

[umbertogriffo]

This PR:

• Fixes a severe bug in saving and loading the merging rules.
• Checks if an element is missing from the vocabulary, raising a warning.
• Drops a useless \n while writing the vocabulary.

The bug

Since we were using BTreeMap we saved and loaded the merging rules (pair and index) not following the order of insertions, causing the following errors while loading the model:

The pairs are not found in the vocabulary since the order of the pairs during the saving and load was wrong.

To fix the issue, we need to use an IndexMap type from the indexmap crate since the standard HashMap in Rust does not maintain insertion order. The issue is not present in the Python version since, from Python 3.7, the built-in dict type maintains insertion order by default.