fix: Verify that unmerged leaves are sorted

[mulmarta]

The RFC requires that unmerged leaves MUST be sorted. Moreover, some of our mechanisms rely on this to work correctly. This PR enforces this when validating the tree on join and inserts the leaf in the correct position when updating the list of unmerged leaves.

I believe that the motivation behind the RFC's requirement is the fact that in a normal execution the list will always be sorted if upon updating the list the new index is simply pushed to the end. The reason is that the list of unmerged leaves of N can only be emptied (when a member in N's subtree is removed, updates or commits) or, when a member is added, extended by the first free leaf index in N's subtree for the new member.

However, I think that one can (maliciously) craft a valid tree where this argument does not hold. For example, a tree with 8 leaves where leaf 3 is an unmerged leaf of its grandparent and leaf 2 is empty.

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

❌ Patch coverage is 95.23810% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.70%. Comparing base (738e250) to head (9b81591).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
mls-rs/src/tree_kem/mod.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #321   +/-   ##
=======================================
  Coverage   88.69%   88.70%           
=======================================
  Files         182      182           
  Lines       28985    29001   +16     
=======================================
+ Hits        25709    25724   +15     
- Misses       3276     3277    +1     

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[tomleavy]

@mulmarta no problem just raising the rust MSRV up to 1.82 so that sort function unsupported warning goes away