dl: add a complete tested shared library generation/usage example

vlib/dl/example/use.v

@@ -0,0 +1,17 @@
+module main
+
+import os
+import dl
+
+type FNAdder = fn (int, int) int
+
+fn main() {
+	library_file_path := os.join_path(os.getwd(), 'library${dl.dl_ext}')
+	handle := dl.open(library_file_path, dl.rtld_lazy)
+	eprintln('handle: ${ptr_str(handle)}')
+	mut f := &FNAdder(0)
+	f = dl.sym(handle, 'add_1')
+	eprintln('f: ${ptr_str(f)}')
+	res := f(1, 2)
+	eprintln('res: $res')
+}