Initial work on stub DSL - found bug in Crystal compiler

```
Module validation failed: Invalid bitcast
  %4 = bitcast i32* %3 to %"(Spectator::Arguments(T, NT) | Nil)", !dbg !16
 (Exception)
  from /crystal/src/int.cr:541:9 in 'finish'
  from /crystal/src/compiler/crystal/codegen/codegen.cr:71:7 in 'codegen'
  from /crystal/src/compiler/crystal/compiler.cr:173:16 in 'compile'
  from /crystal/src/compiler/crystal/command/spec.cr:98:14 in 'spec'
  from /crystal/src/hash.cr:901:11 in '__crystal_main'
  from /crystal/src/crystal/main.cr:115:5 in 'main'
  from src/env/__libc_start_main.c:94:2 in 'libc_start_main_stage2'
Error: you've found a bug in the Crystal compiler. ...
```

I haven't been able to reproduce this in a simplified form.

Passing the `constraint` argument in NullStub#and_return to ValueStub.new triggers the bug.
Removing the `constraint` argument fixes the issue.

spec/spectator/dsl/mocks/stubs_spec.cr

@@ -0,0 +1,11 @@
+require "../../../spec_helper"
+
+Spectator.describe "Stubs DSL" do
+  double(:foobar, foo: 42, bar: "baz")
+  let(dbl) { double(:foobar) }
+
+  specify do
+    allow(dbl).to receive(:foo).and_return(123)
+    expect(dbl.foo).to eq(123)
+  end
+end

src/spectator/dsl/mocks.cr

@@ -127,5 +127,13 @@ module Spectator::DSL
     macro double(**value_methods)
       ::Spectator::LazyDouble.new({{**value_methods}})
     end
+
+    macro allow(stubbable)
+      ::Spectator::Allow.new({{stubbable}})
+    end
+
+    macro receive(method)
+      ::Spectator::NullStub.new({{method.id.symbolize}})
+    end
   end
 end

src/spectator/mocks/allow.cr

@@ -0,0 +1,14 @@
+require "./stub"
+require "./stubbable"
+
+module Spectator
+  struct Allow(T)
+    def initialize(@target : T)
+      {% raise "Target of `allow` must be stubbable (a mock or double)." unless T < Stubbable %}
+    end
+
+    def to(stub : Stub) : Nil
+      @target._spectator_define_stub(stub)
+    end
+  end
+end

src/spectator/mocks/null_stub.cr

@@ -0,0 +1,15 @@
+require "./typed_stub"
+require "./value_stub"
+
+module Spectator
+  # Stub that does nothing and returns nil.
+  class NullStub < TypedStub(Nil)
+    def value : Nil
+      nil
+    end
+
+    def and_return(value)
+      ValueStub.new(method, value, constraint)
+    end
+  end
+end