revert to symbols

Parfait::Words were nice, but endless problems with the fact that when
you write “String” you get a string.
Symbols take care of uniqueness at the same time

lib/virtual/instructions/set.rb

@@ -7,8 +7,6 @@ module Virtual
   class Set < Instruction
     def initialize to , from
       @to = to
-      # hard to find afterwards where it came from, so ensure it doesn't happen
-      raise "From must be slot or constant, not symbol #{from}" if from.is_a? Symbol
       @from = from
     end
     attr_reader :from , :to