chore: allow(unsafe_code) in generated code

[Oyami-Srk]

capnpc-rust starts generating code with unsafe block after #607. Therefore, add #[allow(unsafe_code)] to avoid warnings or errors as much as possible.

However, IMHO, introducing unsafe block to generated code might be an obstacle for projects using #![forbid(unsafe_code)] to guarantee the use of safe Rust only. Some of the projects will need to relax their guarantee to be able to use capnproto-rust, like https://github.com/o1-labs/mina-network-debugger/blob/main/mina-ipc/src/lib.rs.

Additionally, I'm not sure about whether adding these #![allow] to the top of the generated file is a better idea, so I chose to follow the existing one.

P.S., Actually, we already have a #![allow] in generated file:

capnproto-rust/capnpc/src/codegen.rs

Line 2555 in 9c1058b

mod_interior.push(line("#![allow(unused_variables)]"));

^ I'm more than happy to move #![allow] to generated file if this is suitable.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.83%. Comparing base (ab342b3) to head (0260350).
⚠️ Report is 251 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #610      +/-   ##
==========================================
- Coverage   51.64%   50.83%   -0.82%     
==========================================
  Files          69       70       +1     
  Lines       33735    32349    -1386     
==========================================
- Hits        17422    16443     -979     
+ Misses      16313    15906     -407     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[dwrensha]

I opened #611 to discuss this and other possible fixes.

[dwrensha]

Additionally, I'm not sure about whether adding these #![allow] to the top of the generated file is a better idea, so I chose to follow the existing one.

Unfortunately, adding a global attribute to the top of a file pulled in by include!() does not work.

error: an inner attribute is not permitted in this context
 --> /home/dwrensha/src/capnproto-rust/target/debug/build/external-crate-a36be70fac4750a7/out/external_capnp.rs:5:1
  |
5 |   #![allow(unsafe_code)]
  |   ^^^^^^^^^^^^^^^^^^^^^^
6 | / pub mod annot {
7 | |   pub const ID: u64 = 0xfafa2ced66908af5;
8 | |   pub fn get_type() -> ::capnp::introspect::Type { <crate::external_capnp::opts::Owned as ::capnp::introspect::Introspect>::introspect() }
9 | | }
  | |_- the inner attribute doesn't annotate this module

What does work is locally annotating each unsafe instance: #615. I think I prefer that slightly vs this PR.

[dwrensha]

closing in favor of #615