Rollup merge of #77170 - ecstatic-morse:const-fn-ptr, r=oli-obk

Remove `#[rustc_allow_const_fn_ptr]` and add `#![feature(const_fn_fn_ptr_basics)]`

`rustc_allow_const_fn_ptr` was a hack to work around the lack of an escape hatch for the "min `const fn`" checks in const-stable functions. Now that we have co-opted `allow_internal_unstable` for this purpose, we no longer need a bespoke attribute.

Now this functionality is gated under `const_fn_fn_ptr_basics` (how concise!), and `#[allow_internal_unstable(const_fn_fn_ptr_basics)]` replaces `#[rustc_allow_const_fn_ptr]`. `const_fn_fn_ptr_basics` allows function pointer types to appear in the arguments and locals of a `const fn` as well as function pointer casts to be performed inside a `const fn`. Both of these were allowed in constants and statics already. Notably, this does **not** allow users to invoke function pointers in a const context. Presumably, we will use a nicer name for that (`const_fn_ptr`?).

r? @oli-obk

library/std/src/lib.rs

@@ -239,6 +239,7 @@
 #![cfg_attr(not(bootstrap), feature(const_fn_floating_point_arithmetic))]
 #![feature(const_fn_transmute)]
 #![feature(const_fn)]
+#![cfg_attr(not(bootstrap), feature(const_fn_fn_ptr_basics))]
 #![feature(const_ip)]
 #![feature(const_ipv6)]
 #![feature(const_raw_ptr_deref)]