Unify instruction namings also dirs

Was getting confused myself, where it was instruction or instructions, when if the base class was inside or out of dir.
Now dirs are plural, and base class is inside.

lib/slot_machine/instructions/block_yield.rb

@@ -0,0 +1,38 @@
+module SlotMachine
+
+  # A BlockYield calls an argument block. All we need to know is the index
+  # of the argument, and the rest is almost as simple as a SimpleCall
+
+  class BlockYield < Instruction
+    attr :arg_index
+
+    # pass in the source (sol statement) and the index.
+    # The index is the argument index of the block that we call
+    def initialize(source , index)
+      super(source)
+      @arg_index = index
+    end
+
+    def to_s
+      "BlockYield[#{arg_index}] "
+    end
+
+    # almost as simple as a SimpleCall, use a dynamic_jump to get there
+    def to_risc(compiler)
+      return_label = Risc.label("block_yield", "continue_#{object_id}")
+      index = arg_index
+      compiler.build("BlockYield") do
+        next_message! << message[:next_message]
+        return_address! << return_label
+        next_message[:return_address] << return_address
+
+        block_reg! << message["arg#{index}".to_sym]
+
+        message << message[:next_message]
+        add_code Risc::DynamicJump.new("block_yield", block_reg )
+        add_code return_label
+      end
+    end
+  end
+
+end