Rollup merge of #42271 - tinaun:charfromstr, r=alexcrichton

add `FromStr` Impl for `char`

fixes #24939.

is it possible to use pub(restricted) instead of using a stability attribute for the internal error representation? is it needed at all?

src/libstd_unicode/char.rs

@@ -38,6 +38,8 @@ use tables::{conversions, derived_property, general_category, property};
 pub use core::char::{MAX, from_digit, from_u32, from_u32_unchecked};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::char::{EscapeDebug, EscapeDefault, EscapeUnicode};
+#[stable(feature = "char_from_str", since = "1.19.0")]
+pub use core::char::ParseCharError;
 
 // unstable reexports
 #[unstable(feature = "try_from", issue = "33417")]