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/// For unwrapping `Result`s in const contexts, with a default value when it's an error.
/// For unwrapping `Option`s in const contexts, with a default value when it's a `None`.
/// `?`-like macro, which allows optionally mapping errors.
///
/// `?` currently doesn't work in `const fn`s because as of Rust 1.51.0
/// trait methods don't work in `const fn`s.
///
/// # Examples
///
/// ### Basic
///
/// ```rust
/// use konst::try_;
///
/// const OK: Result<&str, u8> = expect_no_whitespace("hello");
/// assert_eq!(OK, Ok("hello"));
///
/// const ERR: Result<&str, u8> = expect_no_whitespace("hello world");
/// assert_eq!(ERR, Err(b' '));
///
///
/// const fn expect_no_whitespace(string: &str) -> Result<&str, u8> {
/// let bytes = string.as_bytes();
/// konst::for_range!{i in 0..bytes.len() =>
/// try_!(assert_not_whitespace(bytes[i]));
/// }
/// Ok(string)
/// }
///
/// const fn assert_not_whitespace(byte: u8) -> Result<(), u8> {
/// if matches!(byte, b'\t' | b'\n' | b'\r' | b' ') {
/// Err(byte)
/// } else {
/// Ok(())
/// }
/// }
///
/// ```
///
/// ### Mapping errors
///
/// ```rust
/// use konst::try_;
///
/// const EVENS: Result<[Even; 4], u32> =
/// array_to_even([0, 2, 4, 6]);
///
/// let new = |n| Even::new(n).unwrap();
/// assert_eq!(EVENS, Ok([new(0), new(2), new(4), new(6)]));
///
///
/// const UNEVEN: Result<[Even; 4], u32> =
/// array_to_even([0, 2, 5, 6]);
///
/// assert_eq!(UNEVEN, Err(5));
///
///
/// const fn array_to_even(arr: [u32; 4]) -> Result<[Even; 4], u32> {
/// let mut ret = [Even::ZERO; 4];
///
/// konst::for_range!{i in 0..4 =>
/// ret[i] = try_!(Even::new(arr[i]), map_err = |e| e.get() );
/// }
///
/// Ok(ret)
/// }
///
/// #[derive(Debug, PartialEq)]
/// pub struct Even(u32);
///
/// impl Even {
/// const ZERO: Even = Even(0);
///
/// pub const fn new(number: u32) -> Result<Self, NotEven> {
/// if number % 2 == 0 {
/// Ok(Even(number))
/// } else {
/// Err(NotEven(number))
/// }
/// }
/// }
///
/// #[derive(Debug, PartialEq)]
/// pub struct NotEven(u32);
///
/// impl NotEven {
/// pub const fn get(&self) -> u32 {
/// self.0
/// }
/// }
///
/// use std::fmt::{self, Display};
///
/// impl Display for NotEven {
/// fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
/// fmt::Debug::fmt(self, f)
/// }
/// }
///
/// impl std::error::Error for NotEven {}
///
/// ```
///
/// `?`-like macro for `Option`s.
///
/// # Example
///
/// ```rust
/// use konst::try_opt;
///
/// const SOME: Option<u8> = sum_u8s(&[3, 5, 8, 13]);
/// assert_eq!(SOME, Some(29));
///
/// const NONE: Option<u8> = sum_u8s(&[3, 5, 8, 13, 240]);
/// assert_eq!(NONE, None);
///
/// const fn sum_u8s(mut nums: &[u8]) -> Option<u8> {
/// let mut sum = 0_u8;
/// while let [first, rem @ ..] = nums {
/// nums = rem;
/// # sum = try_opt!(checked_add(sum, *first));
/// # /*
/// sum = try_opt!(sum.checked_add(*first));
/// # */
/// }
/// Some(sum)
/// }
///
/// # const fn checked_add(l: u8, r: u8) -> Option<u8> {
/// # let (res, overflowed) = l.overflowing_add(r);
/// # if overflowed { None } else { Some(res) }
/// # }
/// ```
///