doc: remove #![feature(const_loop)] and #![feature(const_if_match)] from example code

`const_if_match` and `const_loop` have been stabilized by
<rust-lang/rust#72437>.

Author: yvt

README.md

@@ -27,9 +27,7 @@ Constance is a proof-of-concept of a static RTOS that utilizes Rust's compile-ti
 ## Example
 
 ```rust
-#![feature(const_loop)]
 #![feature(const_fn)]
-#![feature(const_if_match)]
 #![feature(const_mut_refs)]
 use constance::kernel::Task;
 

src/constance_port_std/src/lib.md

@@ -3,9 +3,7 @@ Simulator for running [`::constance`] on a hosted environment
 # Usage
 
 ```rust
-#![feature(const_loop)]
 #![feature(const_fn)]
-#![feature(const_if_match)]
 #![feature(const_mut_refs)]
 
 // Require `unsafe` even in `unsafe fn` - highly recommended