`StatusCode` trait

## Feature Request

### Crates

tonic

### Motivation

The trait can be useful when dealing with error statuses. tonic exposes the `Status::from_error` associated function, which creates a new status with a hardcoded `Code::Unknown` as its status code. With an additional boundary, it would be easy to define the error code.

P.S.
It would also be great to include error backtrace when it's present by iterating though its [sources](https://doc.rust-lang.org/stable/std/error/trait.Error.html#method.source)

### Proposal

Move out the [method](https://docs.rs/tonic/latest/tonic/struct.Status.html#method.code) into a trait
```rust
pub trait ErrorCode {
fn code(&self) -> tonic::Code;
}
```

So that we could

```rust
use tonic::Code;

impl ErrorCode for Error {
fn code(&self) -> Code {
match self {
Error::Two(_) => Code::FailedPrecondition,
_ => Code::Internal,
}
}
}
```

### Alternatives

It would be great to implement this via [std::process::Termination](https://doc.rust-lang.org/stable/std/process/trait.Termination.html) trait instead, but getting the code as `i32` from [std::process::ExitCode](https://doc.rust-lang.org/stable/std/process/struct.ExitCode.html) is not stable yet.