Source hashing and reduced generated code

• The generated file now contains a hash of all the files that needle actually looked at. This allows optimizations where we can decide to skip recompiling the generated file if nothing has changed since last time.
• The generated code is now optimized for the case where a specific needle scope may be attached at various different points in the tree. Until now, this lead to a provider class being generated for every unique path. Now, paths that have similar providers use a base class to shared most of the code.