stdlib: Use Conlang module layout

stdlib/std.cl

@@ -0,0 +1,12 @@
+//! # The Conlang Standard Library
+
+pub mod preamble {
+    pub use super::{num::*, str::str};
+}
+
+pub mod num;
+
+pub mod str;
+
+#[cfg("test")]
+mod test;