Refactor: utils::buffer

[rrybarczyk]

Background

This task is an outcome of the protocols Rust docs issues tracked in #845.

While documenting utils::buffer in #1184, areas of potential code debt were identified. This issue servers as a place to list out these items to then be addressed in an organized manner. The initial Rust documentation effort was an immediate action, while a refactoring (which implies breaking API changes) is not so urgent priority, so for now we should leave this in the backlog for an appropriate moment in the future.

[rrybarczyk]

Questions:

• Why is buffer_sv2 imported in the roles/pool, /jd-server, /translator, and
  test-utils/mining-device if not used?
• slice.rs impl Serialize for Slice: why is this implemented if it simply panics when called? is
  it for feature flag compatibility, or some way to indicate that the serialization of Slice is not
  supported, or something else?
• slice.rs: Why is IGNORE_INDEX set to 59?
• slice.rs, SharedState::toogle debug mode: What is the mode? what are all the possible modes?
  Just reading or writing? Processing? Other?
• pool_back.rs try_clear_head, why do we not return a BufferFromSystemMemory and the
  Err(PoolMode::Alloc) like we do in the PoolFront if-else stmt block?

[rrybarczyk]

Suggestions:

• Rename SharedState::toogle() to toggle.
• slice.rs: Import core::ops::Index and core::ops::IndexMut directly at top of module.
• lib.rs: Rename Buffer::get_data_by_ref -> Buffer::get_data_by_ref_mut and
  Buffer::get_data_by_ref_ -> Buffer::get_data_by_ref
• buffer.rs: Handing tests towards the end of the file, followed by not-test logic. Needs to be
  moved to the bottom of the file inside a mod tests block.

[rrybarczyk]

Why is AEADBuffer handled in this way:

lib.rs exports aes_gcm::aead::Buffer as AeadBuffer. This comes from
here.

[Fi3]

Why is AEADBuffer handled in this way:

lib.rs exports aes_gcm::aead::Buffer as AeadBuffer. This comes from here.

buffer pool is used also for encrypt and decrypt and to do that in place we need that trait