refactor: migrate examples to import Std::Io instead of raw extern func

- Create stdlib/Std/Io.bux with extern func declarations
- Rename C shim functions to short names (PrintLine, PrintInt, etc.)
- Update all 9 examples to use import Std::Io::{PrintLine, PrintInt};
- Remove manual extern func Std_Io_* declarations from examples

examples/methods.bux

@@ -1,6 +1,6 @@
 // Methods - Struct with methods using extend
-extern func Std_Io_PrintLine(s: String);
-extern func Std_Io_PrintInt(n: int);
+import Std::Io::{PrintLine, PrintInt};
+
 
 struct Rectangle {
     width: int;
@@ -20,19 +20,19 @@ extend Rectangle {
 func Main() -> int {
     let rect: Rectangle = Rectangle { width: 10, height: 5 };
     
-    Std_Io_PrintLine("Rectangle:");
-    Std_Io_PrintLine("Width = ");
-    Std_Io_PrintInt(rect.width);
-    Std_Io_PrintLine("");
-    Std_Io_PrintLine("Height = ");
-    Std_Io_PrintInt(rect.height);
-    Std_Io_PrintLine("");
-    Std_Io_PrintLine("Area = ");
-    Std_Io_PrintInt(rect.Area());
-    Std_Io_PrintLine("");
-    Std_Io_PrintLine("Perimeter = ");
-    Std_Io_PrintInt(rect.Perimeter());
-    Std_Io_PrintLine("");
+    PrintLine("Rectangle:");
+    PrintLine("Width = ");
+    PrintInt(rect.width);
+    PrintLine("");
+    PrintLine("Height = ");
+    PrintInt(rect.height);
+    PrintLine("");
+    PrintLine("Area = ");
+    PrintInt(rect.Area());
+    PrintLine("");
+    PrintLine("Perimeter = ");
+    PrintInt(rect.Perimeter());
+    PrintLine("");
     
     return 0;
 }