Use Box for Wrapper implementation

This won't raise on invalid casts.
It should reduce the amount of instantiated types and methods.

spec/spectator/wrapper_spec.cr

@@ -10,9 +10,4 @@ Spectator.describe Spectator::Wrapper do
     wrapper = described_class.new(Int32)
     expect(wrapper.get { Int32 }).to eq(Int32)
   end
-
-  it "raises on invalid cast" do
-    wrapper = described_class.new(42)
-    expect { wrapper.get(String) }.to raise_error(TypeCastError)
-  end
 end

src/spectator/wrapper.cr

@@ -9,18 +9,17 @@ module Spectator
   # value = wrapper.get(String)
   # ```
   struct Wrapper
-    @value : TypelessValue
+    @pointer : Void*
 
     # Creates a wrapper for the specified value.
     def initialize(value)
-      @value = Value.new(value)
+      @pointer = Box.box(value)
     end
 
     # Retrieves the previously wrapped value.
     # The *type* of the wrapped value must match otherwise an error will be raised.
     def get(type : T.class) : T forall T
-      value = @value.as(Value(T))
-      value.get
+      Box(T).unbox(@pointer)
     end
 
     # Retrieves the previously wrapped value.
@@ -35,29 +34,7 @@ module Spectator
     # type = wrapper.get { Int32 } # Returns Int32
     # ```
     def get(& : -> T) : T forall T
-      value = @value.as(Value(T))
-      value.get
-    end
-
-    # Base type that generic types inherit from.
-    # This provides a common base type,
-    # since Crystal doesn't support storing an `Object` (yet).
-    # Instances of this type must be downcast to `Value` to be useful.
-    private abstract class TypelessValue
-    end
-
-    # Generic value wrapper.
-    # Simply holds a value and inherits from `TypelessValue`,
-    # so that all types of this class can be stored as one.
-    private class Value(T) < TypelessValue
-      # Creates the wrapper with the specified value.
-      def initialize(@value : T)
-      end
-
-      # Retrieves the wrapped value.
-      def get : T
-        @value
-      end
+      Box(T).unbox(@pointer)
     end
   end
 end