make ParseIntError, ParseFloatError and ParseBoolError reusable (public)

I'd like to reuse ParseIntError to write a wrapper for the as keyword to make sure the new value matches the old one. It could look like this:

fn convert_usize_to_isize(from: usize) -> Result<isize, ParseIntError> {
    let to = from as isize;
    match (to as usize) == from {
        true => Ok(to),
        false => Err(ParseIntError { kind: IntErrorKind::Overflow }),
    }
}

Furthermore, ParseIntError and ParseFloatError would be useful if one decides to create his own integer or float data type.

To make this work, ParseIntError's kind field, ParseFloatError's kind field and both enums IntErrorKind and FloatErrorKind have to be public.
Whether this should affect ParseBoolError should be discussed.

Start by reviewing the proposed public API changes in the issue, including the error types, their kind fields, and the IntErrorKind and FloatErrorKind enums. Decide whether ParseBoolError belongs in the change, then define the public surface and confirm that the conversion example and custom numeric types can reuse these errors.