## Summary
Changed firm block channel to use boxed `ReconstructedBlock`.

## Background
A large error variant lint was triggered when running clippy with Rust
1.83.0 in #1857 due to send errors returning the whole block. Large enum
variants should be avoided because enums are only as small as their
largest variant:
https://rust-lang.github.io/rust-clippy/master/index.html#large_enum_variant.
Changing the channel to consist of a boxed block instead solves this
problem at its source.

## Changes
- Changed firm block channel to use boxed `ReconstructedBlock` instead
of it being unboxed.

## Testing
Passing all tests.

## Changelogs
No updates needed.

## Breaking Changes
Overridden code freeze since this is a very small, non breaking change
that shouldn't have any bearing since our previous audit.

## Related Issues
closes #1858