[wip] introduce BuildLibrary and make overlay first-class

examples/tutorial/README.md

@@ -20,10 +20,10 @@ Contents
     * Use `Pather` to snap ports together into a circuit
     * Check for dangling references
 - [library](library.py)
-    * Continue from `devices.py` using a lazy library
-    * Create a `LazyLibrary`, which loads / generates patterns only when they are first used
+    * Continue from `devices.py` by declaring a mixed library with `BuildLibrary`
+    * Import source-backed GDS cells and register python-generated recipes together
+    * Call `build()` to produce a normal library for downstream `Pather` usage and writing
     * Explore alternate ways of specifying a pattern for `.plug()` and `.place()`
-    * Design a pattern which is meant to plug into an existing pattern (via `.interface()`)
 - [pather](pather.py)
     * Use `Pather` to route individual wires and wire bundles
     * Use `AutoTool` to generate paths