Add Parser implementation for nom::combinator::value

[althonos]

Salut Geoffroy!

This PR adds a Parser implementation for nom::combinator::value, so that the underlying parser is always run in Check mode and the parser value is never emitted, as it would be replaced by the user-provided value anyway.

The old code is doing:

pub fn value<I, O1: Clone, E: ParseError<I>, F>(
 where
   F: Parser<I, Error = E>,
 {
    parser.map(move |_| val.clone())
 }

which basically delegates to using the Parser::map implementation. Parser::map maps the closure to the output of the parser, which means that the parser output must always be emitted. But since in the case of value we are gonna ignore the result anyway, we can actually always run the underlying parser in Check mode.

I added a dedicated Value<F, O1> struct with he following implementation to avoid emitting the inner parser value (similarly to preceded, etc.):

impl<I, O1, E, F> Parser<I> for Value<F, O1>
where
  F: Parser<I, Error = E>,
  O1: Clone,
  E: ParseError<I>,
{
  type Output = O1;
  type Error = E;
  fn process<OM: OutputMode>(
      &mut self,
      input: I,
    ) -> PResult<OM, I, Self::Output, Self::Error> {
      self.parser.process::<OutputM<Check, OM::Error, OM::Incomplete>>(input)
        .map(|(i, _)| ( i, OM::Output::bind(|| self.val.clone()) ))
    }
}

I don't think the value combinator is tested anywhere outside of the doctests, let me know if I should add some tests as well.