feat: nodes accessor for lean-imt

[brech1]

Description

This PR adds a nodes method to the LeanIMT struct, exposing the internal nodes structure as a slice (&[Vec<[u8; N]>]).

It also fixes minor clippy warnings by allowing manual_div_ceil and reformat the example in lib.rs.

• What kind of change? Feature addition
• Current behavior: No method exists to access the internal nodes structure of LeanIMT.
• New behavior: The nodes method returns a reference to the internal nodes, allowing read-only access.
• Breaking change? No.

Related Issue(s)

Resolves #76

Other information

The change is minimal and does not affect existing functionality. No new tests were added as the method is a simple accessor, but existing tests pass.

Checklist

• I have read and understand the contributor guidelines and code of conduct.
• I have performed a self-review of my code.
• I have commented my code, particularly in hard-to-understand areas.
• My changes generate no new warnings.
• I have run cargo fmt without getting any errors.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.