anyhow_trace 0.1.3

Macro which adds source location as context to anyhow errors
Documentation 
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This macro re-writes try expressions to add a call to `anyhow::Context::with_context` which adds the
source location of the try expression as context to the error.

This creates something that is similar to a backtrace, but is actually the path the error propagates
through instead. This can be preferable to using anyhow's backtrace feature because it doesn't rely
on unwinding the stack or debugging symbols being compiled in.

Here is an example
```rust
use anyhow_trace::anyhow_trace;
use anyhow::{bail, Result};

fn foo_err() -> Result<()> {
    bail!("this is foo error")
}

struct FooBar;

#[anyhow_trace]
impl FooBar {
    fn thing() -> Result<()> {
        fn im_inside() -> Result<()> {
            foo_err()?;
            Ok(())
        }
        im_inside()?;
        Ok(())
    }
}

#[anyhow_trace]
fn inner() -> Result<()> {
    FooBar::thing()?;
    Ok(())
}

#[anyhow_trace]
fn main() -> Result<()> {
    inner()?;
    Ok(())
}
```

This prints out the following:
```
Error: main at examples/example.rs:30:5

Caused by:
    0: inner at examples/example.rs:24:5
    1: FooBar::thing at examples/example.rs:17:9
    2: FooBar::thing::im_inside at examples/example.rs:14:13
    3: this is foo error
```