Don't use std:: paths in syntax extensions when compiling a #![no_std] crate

Fixes #16803.
Fixes #14342.
Fixes half of #21827 -- slice syntax is still broken.

src/libsyntax/std_inject.rs

@@ -37,7 +37,7 @@ pub fn maybe_inject_prelude(krate: ast::Crate) -> ast::Crate {
     }
 }
 
-fn use_std(krate: &ast::Crate) -> bool {
+pub fn use_std(krate: &ast::Crate) -> bool {
     !attr::contains_name(&krate.attrs[], "no_std")
 }