Remove 2 frames of noise from 'context' backtraces

[dtolnay]

Repro:

use anyhow::Context;

fn main() -> anyhow::Result<()> {
    let result = Err(std::fmt::Error);
    result.context("...")
}

Before:

0: <E as anyhow::context::ext::StdError>::ext_context
          at /git/anyhow/src/context.rs:27:29
1: anyhow::context::<impl anyhow::Context<T,E> for core::result::Result<T,E>>::context::{{closure}}
          at /git/anyhow/src/context.rs:50:30
2: core::result::Result<T,E>::map_err
          at /rustc/4b8f4319954ff2642690b9e5cbe4af352d095bf6/library/core/src/result.rs:861:27
3: anyhow::context::<impl anyhow::Context<T,E> for core::result::Result<T,E>>::context
          at /git/anyhow/src/context.rs:50:9
4: testing::main
          at ./src/main.rs:5:5

After:

0: <E as anyhow::context::ext::StdError>::ext_context
          at /git/anyhow/src/context.rs:27:29
1: anyhow::context::<impl anyhow::Context<T,E> for core::result::Result<T,E>>::context
          at /git/anyhow/src/context.rs:52:31
2: testing::main
          at ./src/main.rs:5:5