/// Returns a function that discards its argument and always returns `r`.
///
/// ```
/// use toad_common::*;
///
/// fn try_get_string() -> Result<String, std::io::Error> {
/// # Ok("".into())
/// }
///
/// fn do_stuff() -> Result<String, std::io::Error> {
/// try_get_string().map(const_("it worked!".to_string())) // equivalent to:
/// .map(|_| "it worked!".to_string())
/// }
/// ```
/// A function that discards its argument and always returns unit `()`
///
/// ```
/// use toad_common::*;
///
/// fn try_get_string() -> Result<String, std::io::Error> {
/// # Ok("".into())
/// }
///
/// fn do_stuff() -> Result<(), std::io::Error> {
/// try_get_string().map(ignore) // equivalent to:
/// .map(|_| ())
/// }
/// ```