Add Core.PrintStr and a "hello world" example (#6493)

Thanks to @ammaralassal for #6329 which did the heavy lifting to make
this possible!

Co-authored-by: Josh L <josh11b@users.noreply.github.com>

core/io.carbon

@@ -13,6 +13,16 @@ import library "prelude";
 // TODO: Consider rewriting using native support once library support exists.
 fn Print(x: i32) = "print.int";
 fn PrintChar(x: char) -> i32 = "print.char";
+
+fn PrintStr(msg: str) {
+  let size: i64 = msg.Size() as i64;
+  var i: i64 = 0;
+  while (i < size) {
+    PrintChar(msg[i]);
+    ++i;
+  }
+}
+
 // TODO: Return an `Optional(char)` instead of `i32`.
 fn ReadChar() -> i32 = "read.char";
 

examples/BUILD

@@ -8,3 +8,8 @@ carbon_binary(
     name = "sieve",
     srcs = ["sieve.carbon"],
 )
+
+carbon_binary(
+    name = "hello_world",
+    srcs = ["hello_world.carbon"],
+)

examples/hello_world.carbon

@@ -0,0 +1,9 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+import Core library "io";
+
+fn Run() {
+  Core.PrintStr("Hello world!\n");
+}