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/parfait/class.rb

@@ -1,5 +1,5 @@
 # Class is mainly a list of methods with a name.
-# The methods are untyped, sis VoolMethod.
+# The methods are untyped, sis SolMethod.
 
 # The memory layout of an object is determined by the Type (see there).
 # The class carries the "current" type, ie the type an object would be if you
@@ -11,7 +11,7 @@
 
 # An Object carries the data for the instance variables it has.
 # The Type lists the names of the instance variables
-# The Class keeps a list of instance methods, these have a name and (vool) code
+# The Class keeps a list of instance methods, these have a name and (sol) code
 # Each type in turn has a list of CallableMethods that hold binary code
 
 module Parfait