Implement TryFrom<char> for u8

Previously suggested in https://github.com/rust-lang/rfcs/issues/2854.

It makes sense to have this since `char` implements `From<u8>`. Likewise
`u32`, `u64`, and `u128` (since #79502) implement `From<char>`.

library/core/src/char/mod.rs

@@ -544,3 +544,15 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         fmt::Display::fmt(&self.0, f)
     }
 }
+
+/// The error type returned when a checked char conversion fails.
+#[stable(feature = "u8_from_char", since = "1.59.0")]
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub struct TryFromCharError(pub(crate) ());
+
+#[stable(feature = "u8_from_char", since = "1.59.0")]
+impl fmt::Display for TryFromCharError {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        "unicode code point out of range".fmt(fmt)
+    }
+}