chore: make it compile with Rust 1.71

[vmx]

The current version won't compile with Rust 1.71:

error: unnecessary qualification
    --> src/merkle.rs:2021:17
     |
2021 |     iter: <I as std::iter::IntoIterator>::IntoIter,
     |                 ^^^^^^^^^^^^^^^^^^^^^^^
     |
note: the lint level is defined here
    --> src/lib.rs:72:5
     |
72   |     unused_qualifications,
     |     ^^^^^^^^^^^^^^^^^^^^^

error: could not compile `merkletree` (lib) due to previous error

This commit fixes that error.