Fix PatriciaTree::iter_prefix_mut() bug

[sile]

This PR fixes the bug mentioned in #43 (comment).

Copilot Summary

This pull request includes changes to the src/map.rs file, focusing on adding a new field to the IterMut struct and modifying the Iterator implementation for IterMut. Additionally, a previously ignored test has been re-enabled.

Enhancements to IterMut struct and iterator implementation:

• src/map.rs: Added a new field key_offset to the IterMut struct to store the length of the key and initialized it in the new method.
• src/map.rs: Updated the next method in the Iterator implementation for IterMut to use key_offset when truncating key_bytes.

Testing improvements:

• src/map.rs: Re-enabled the issue42_iter_prefix_mut test by removing the #[ignore] attribute.