- Removes closures and function calls for types that implement default:
```rust
// Change
let _my_str = get_str().unwrap_or(String::new);
// To
let _my_str = get_str().unwrap_or_default();
```
- Uses the `.cloned()/.copied()` methods where possible
- Use function pointer instead of simple closure
May improve performace, as closures generate more code, and this might
unlock some inlining opportunities.
Invalid option names, or values that evaluate as nil (such as the `foo`
above: that's treated as a global variable reference, but `foo` isn't a
defined global and evaluates as `nil`) will now cause the program
startup to error out with an actionable error message.
Previously, the invalid config name would generate a warning, and the
invalid value would silently have no effect as it has the same effect as
omitting the named value and leaving it as its default value.
I think these cases should both immediately error out and stop
further processing, so that's what we're doing.
This commit also adds support for adding:
```
#[dynamic(deprecated = "use newer option instead")]
pub some_config_value: bool,
```
but not options currently use this.
