Rollup merge of #89258 - est31:const_char_convert, r=oli-obk

Make char conversion functions unstably const

The char conversion functions like `char::from_u32` do trivial computations and can easily be converted into const fns. Only smaller tricks are needed to avoid non-const standard library functions like `Result::ok` or `bool::then_some`.

Tracking issue: https://github.com/rust-lang/rust/issues/89259

library/core/src/lib.rs

@@ -105,6 +105,7 @@
 #![feature(const_bigint_helper_methods)]
 #![feature(const_caller_location)]
 #![feature(const_cell_into_inner)]
+#![feature(const_char_convert)]
 #![feature(const_discriminant)]
 #![feature(const_eval_select)]
 #![feature(const_float_bits_conv)]