Macro `const_panic`

macro const_panic {
    ($const_msg:literal, $runtime_msg:literal, $($arg:ident : $ty:ty = $val:expr),* $(,)?) => { ... },
    ($const_msg:literal, $runtime_msg:literal, $($arg:ident : $ty:ty),* $(,)?) => { ... },
}

Helper macro for panicking in a const fn. Invoke as:

core::macros::const_panic!("boring message", "flavored message {a} {b:?}", a: u32 = foo.len(), b: Something = bar);

where the first message will be printed in const-eval, and the second message will be printed at runtime.