Make Error::from_std public

Make `Error::from_std(impl StdError, Option)` public so a user can control whether capture backtrace or not.

Alternatively, `Error` may have constructors with an option to capture backtrace, like:

```
impl Error {
pub fn with_backtrace(impl StdError) { ... }
pub fn without_backtrace(impl StdError) { ... }
}
```

These are useful for:
* tests of error handling, like what is printed exactly
* make some errors cheaper or more detailed regardless of global setup