rust: add `build_error` crate

2022-12-04T00:59:16Z

The `build_error` crate provides a function `build_error` which will panic at compile-time if executed in const context and, by default, will cause a build error if not executed at compile time and the optimizer does not optimise away the call. The `CONFIG_RUST_BUILD_ASSERT_ALLOW` kernel option allows to relax the default build failure and convert it to a runtime check. If the runtime check fails, `panic!` will be called. Its functionality will be exposed to users as a couple macros in the `kernel` crate in the following patch, thus some documentation here refers to them for simplicity. Signed-off-by: Gary Guo Reviewed-by: Wei Liu [Reworded, adapted for upstream and applied latest changes] Signed-off-by: Miguel Ojeda

kernel/rust/build_error.rs, branch 0x221E-v0.0.1-v6.19

rust: add `build_error` crate