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

stdlib/Std/Io.bux

@@ -0,0 +1,10 @@
+module Std::Io {
+
+extern func PrintLine(s: String);
+extern func Print(s: String);
+extern func PrintInt(n: int);
+extern func PrintFloat(f: float64);
+extern func PrintBool(b: bool);
+extern func ReadLine() -> String;
+
+}