Add new Orb.Instruction.Const

Dedicated to const, which makes it easier to work with and transform. I’ll remove the old paths through Orb.Instruction next

lib/orb/constants.ex

@@ -141,12 +141,8 @@ defmodule Orb.Constants do
 
     defimpl Orb.ToWat do
       def to_wat(%Orb.Constants.NulTerminatedString{memory_offset: memory_offset}, indent) do
-        [
-          indent,
-          "(i32.const ",
-          to_string(memory_offset),
-          ")"
-        ]
+        Orb.Instruction.Const.new(:i32, memory_offset)
+        |> Orb.ToWat.to_wat(indent)
       end
     end
   end

lib/orb/instruction/const.ex

@@ -0,0 +1,19 @@
+defmodule Orb.Instruction.Const do
+  @moduledoc false
+  defstruct [:push_type, :value]
+
+  def new(push_type, value) when push_type in ~w(i32 i64 f32 f64)a do
+    %__MODULE__{push_type: push_type, value: value}
+  end
+
+  defimpl Orb.ToWat do
+    def to_wat(%Orb.Instruction.Const{push_type: push_type, value: value}, indent) do
+      [
+        indent,
+        ["(", Atom.to_string(push_type), ".const "],
+        to_string(value),
+        ")"
+      ]
+    end
+  end
+end