Fixed Never Type Fallback being () warning

[sigurpol]

This prevents the future hard error that would occur in Rust 2024.

cargo report future-incompatibilities --id 1 --package trie-db@0.30.0
The following warnings were discovered during the build. These warnings are an
indication that the packages contain code that will become an error in a
future release of Rust. These warnings typically cover changes to close
soundness problems, unintended or undocumented behavior, or critical problems
that cannot be fixed in a backwards-compatible fashion, and are not expected
to be in wide use.

Each warning should contain a link for more information on what the warning
means and how to resolve it.

To solve this problem, you can try the following approaches:

- If the issue is not solved by updating the dependencies, a fix has to be
implemented by those dependencies. You can help with that by notifying the
maintainers of this problem (e.g. by creating a bug report) or by proposing a
fix to the maintainers (e.g. by creating a pull request):

  - trie-db@0.30.0
  - Repository: https://github.com/paritytech/trie
  - Detailed warning command: `cargo report future-incompatibilities --id 1 --package trie-db@0.30.0`

- If waiting for an upstream fix is not an option, you can use the `[patch]`
section in `Cargo.toml` to use your own version of the dependency. For more
information, see:
https://doc.rust-lang.org/cargo/reference/overriding-dependencies.html#the-patch-section

The package `trie-db v0.30.0` currently triggers the following future incompatibility lints:
> warning: this function depends on never type fallback being `()`
>    --> /Users/paolo/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/trie-db-0.30.0/src/node.rs:206:2
>     |
> 206 | /     pub fn to_owned_node<L: TrieLayout>(
> 207 | |         &self,
> 208 | |     ) -> Result<NodeOwned<TrieHash<L>>, TrieHash<L>, CError<L>> {
>     | |_______________________________________________________________^
>     |
>     = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in Rust 2024 and in a future release in all editions!
>     = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2024/never-type-fallback.html>
>     = help: specify the types explicitly
> note: in edition 2024, the requirement `!: FromIterator<()>` will fail

Since we are there, fixed also lifetime syntax warnings and dead code warnings.