Rename directories

I revised the naming scheme for projects containing both AppleCore and
handwritten assembly files:

1. Before I was using 'ac' and 'avm' to refer to two different things:

   a. AppleCore vs. assembly source files.

   b. AppleCore vs. assembly files in general (including AVM files
      generated from AC source files).

   For example, Lib/ac contained subdirectories 'ac' (for the
   AppleCore source files) and 'avm' (for the compiler-generated AVM
   files); and Lib/avm contained handwritten assembly.

2. Since in retrospect this dual use seemed confusing, I changed the
   naming scheme:

   - 'ac' and 'avm' refer only to directories that contain *.ac or
     *.avm files, respectively.

   - In cases where I need separate directories for AppleCore and
     assembly source files, I use the names AppleCore and Assembly.

   For example, Lib/AppleCore now contains 'ac' for source files and
   'avm' for generated files; and Lib/Assembly contains 'avm' for
   source files.

   Hopefully this will be clearer!