Suggestion: implementation of FromPest for std::str::FromStr

[DPDmancul]

Adding this implementation in https://github.com/pest-parser/ast/blob/master/src/lib.rs:

impl<'pest, T: std::str::FromStr> FromPest<'pest> for T {
    type Rule = /* Rule */;
    type FatalError = T::Err;
    fn from_pest(pest: &mut Pairs<'pest, Self::Rule>) -> Result<T, ConversionError<Self::FatalError>> {
        pest.next().ok_or(ConversionError::NoMatch)?.as_str().parse::<T>().ok_or_else(|e|ConversionError::Malformed(e))
    }
}

simplifies the parsing of values (which is a common task) from

#[derive(Debug, FromPest)]
#[pest_ast(rule(Rule::field))]
pub struct Field {
    #[pest_ast(outer(with(span_into_str), with(str::parse), with(Result::unwrap)))]
    pub value: f64,
}

to

#[derive(Debug, FromPest)]
#[pest_ast(rule(Rule::field))]
pub struct Field {
    pub value: f64,
}

This implementation must be inserted in the crate since it cannot be inserted in the final code being both the FromPest and FromStr trials not part of the final code and implementation is allowed only for trials and types defined in the crate.

The problem is how to pass the Rule type.