Use out_is_terminal

This removes some spurious unsafe and some imports.

Note: We don't use it in `test`, because that can be asked to check
arbitrary file descriptors, while this only checks stdout specifically.

fish-rust/src/builtins/set_color.rs

@@ -78,8 +78,7 @@ fn print_colors(
 
     let term = curses::term();
     for color_name in args {
-        // Safety: isatty cannot fail.
-        if !streams.out_is_redirected && unsafe { libc::isatty(libc::STDOUT_FILENO) == 1 } {
+        if streams.out_is_terminal() {
             if let Some(term) = term.as_ref() {
                 print_modifiers(outp, term, bold, underline, italics, dim, reverse, bg);
             }

fish-rust/src/builtins/type.rs

@@ -1,6 +1,3 @@
-use libc::isatty;
-use libc::STDOUT_FILENO;
-
 use super::prelude::*;
 use crate::ffi::{builtin_exists, colorize_shell};
 use crate::function;
@@ -138,7 +135,7 @@ pub fn r#type(
                             props.annotated_definition(arg)
                         ));
 
-                        if !streams.out_is_redirected && unsafe { isatty(STDOUT_FILENO) == 1 } {
+                        if streams.out_is_terminal() {
                             let col = colorize_shell(&def.to_ffi(), parser.pin()).from_ffi();
                             streams.out.append(col);
                         } else {