Macro impl_enum

macro_rules! impl_enum {
    ($x:path {$($($k:pat)|* => $v:expr)+ }) => { ... };
}

Implement FromConfig for enums.

impl_enum!(Ordering{
    "lt" | "less" => Ordering::Less
    "eq" | "equal" => Ordering::Equal
    "gt" | "greater" => Ordering::Greater
});