Rename Vool to Sol

Simple is really the descriptive name for the layer
Sure, it is "virtual" but that is not as important as the fact that it is simple (or simplified)
Also objct (based really) is better, since orientated implies it is a little like that, but only orientated, not really it. Sol only has objects, nothing else
Just cause i was renaming anyway

lib/ruby/ruby_compiler.rb

@@ -22,7 +22,7 @@ module Ruby
   #
   # As a second step, it extracts classes, methods, ivars and locals.
   #
-  # The next step is then to go to the vool level, which is
+  # The next step is then to go to the sol level, which is
   # simpler, and then finally to compile
   # to the next level down, SlotMachine (Minimal Object Machine)
   class RubyCompiler < AST::Processor