Add "pure" procedure types

examples/demo/demo.odin

@@ -1961,6 +1961,22 @@ relative_data_types :: proc() {
 	fmt.println(rel_slice[1]);
 }
 
+pure_procedures :: proc() {
+	fmt.println("\n#pure procedures");
+
+	square :: proc "pure" (x: int) -> int {
+		return x*x + 1;
+	}
+
+	do_math :: proc "pure" (x: int) -> int {
+		// Only "pure" procedure calls are allowed within a "pure" procedure
+		return square(x) + 1;
+	}
+
+	x := do_math(5);
+	fmt.println("do_math(5) ==", x);
+}
+
 main :: proc() {
 	when true {
 		the_basics();
@@ -1993,5 +2009,6 @@ main :: proc() {
 		union_maybe();
 		explicit_context_definition();
 		relative_data_types();
+		pure_procedures();
 	}
 }