clippy: Fix lint about TryFrom & TryInto

[hackaugusto]

The 2021 prelude includes TryFrom and TryInto [1]. And recently the nigtlhy channel started linting for this duplicated imports and emitting warnings. This removes the redundant imports.

1: https://doc.rust-lang.org/core/prelude/rust_2021/index.html

---

Here is an example warning:

warning: the item `TryInto` is imported redundantly
   --> examples/src/lamport/signature.rs:8:26
    |
8   | use std::{cmp::Ordering, convert::TryInto};
    |                          ^^^^^^^^^^^^^^^^
    |
   ::: /Users/hack/.rustup/toolchains/nightly-aarch64-apple-darwin/lib/rustlib/src/rust/library/std/src/prelude/mod.rs:129:13
    |
129 |     pub use core::prelude::rust_2021::*;
    |             ------------------------ the item `TryInto` is already defined here
    |
    = note: `#[warn(unused_imports)]` on by default

[hackaugusto]

The CI failure is related, in the sense the warnings seem to have started with the same change for TryFrom and TryInto. However, those require additional changes from std/no_std support, and I will fix in a follow up.

[irakliyk]

Closing this as these changes were also included in #250.