Make raw standard stream constructors const

library/std/src/sys/unsupported/stdio.rs

@@ -5,7 +5,7 @@ pub struct Stdout;
 pub struct Stderr;
 
 impl Stdin {
-    pub fn new() -> Stdin {
+    pub const fn new() -> Stdin {
         Stdin
     }
 }
@@ -17,7 +17,7 @@ impl io::Read for Stdin {
 }
 
 impl Stdout {
-    pub fn new() -> Stdout {
+    pub const fn new() -> Stdout {
         Stdout
     }
 }
@@ -33,7 +33,7 @@ impl io::Write for Stdout {
 }
 
 impl Stderr {
-    pub fn new() -> Stderr {
+    pub const fn new() -> Stderr {
         Stderr
     }
 }