clippy: type_complexity

[brooksprumo]

Problem

New clippy lints when upgrading to rust 1.84.0.

warning: very complex type used. Consider factoring parts into `type` definitions
   --> ledger/src/blockstore.rs:679:10
    |
679 |     ) -> Result<impl Iterator<Item = (Box<[u8]>, Box<[u8]>)> + '_> {
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#type_complexity
    = note: `#[warn(clippy::type_complexity)]` on by default

warning: very complex type used. Consider factoring parts into `type` definitions
   --> ledger/src/blockstore.rs:689:10
    |
689 |     ) -> Result<impl Iterator<Item = ((u64, u64), Box<[u8]>)> + '_> {
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#type_complexity

warning: very complex type used. Consider factoring parts into `type` definitions
   --> ledger/src/blockstore.rs:701:10
    |
701 |     ) -> Result<impl Iterator<Item = ((u64, u64), Box<[u8]>)> + '_> {
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#type_complexity

Summary of Changes

Resolve the lints by using the suggestions from clippy and checking the code.

Partially fixes #4380

[brooksprumo]

If you would prefer an alternate solution (e.g. creating a proper type to return here), please feel free to push it to this PR.