Right now
- `default_value="something"` is a raw method
- `default_value` uses native types
This commit splits the meanings
- `default_value="something"` is a raw method
- `default_value_t` uses `T::default()`
- `default_value_t=expr` uses an expression that evaluates to `T`
This is meant to mirror the `value_of` / `value_of_t` API.
At the moment, this is limited to `T: Display` to work with clap's
default system. Something we can look at in the future is a way to
loosen that restriction. One quick win is to specialize when `arg_enum`
is set. The main downside is complicating the processing of attributes
because it then means we need some processed before others.
Since this builds on `clap`s existing default system, this also means
users do not get any performance gains out of using `default_value_t`,
since we still need to parse it but we also need to convert it to a
string.
Fixes#1694
