Fix unbox.any for GUID type

- Add GUID to WellKnownTypes.
- Add code to PerformUnboxing to check for valid GUID unboxing.

src/CLR/Core/CLR_RT_HeapBlock.cpp

@@ -918,6 +918,21 @@ HRESULT CLR_RT_HeapBlock::PerformUnboxing(const CLR_RT_TypeDef_Instance &cls)
             // No luck. The types in src object and specified by cls are different. Need to throw exceptioin.
             NANOCLR_SET_AND_LEAVE(CLR_E_INVALID_CAST);
         }
+
+        // now check edge cases
+
+        //////////////////////////////////////////////////////////
+        // GUID: can't cast to anything except another GUID object
+        CLR_RT_TypeDescriptor srcTypeDes;
+        NANOCLR_CHECK_HRESULT(srcTypeDes.InitializeFromObject(*src));
+
+        CLR_RT_TypeDef_Instance& inst = srcTypeDes.m_handlerCls;
+
+        if (inst.m_data == g_CLR_RT_WellKnownTypes.m_Guid.m_data)
+        {
+            // can't cast GUID class to anything else except another GUID
+            NANOCLR_SET_AND_LEAVE(CLR_E_INVALID_CAST);
+        }
     }
 
     if (cls.m_target->dataType == DATATYPE_VALUETYPE)

src/CLR/Core/TypeSystem.cpp

@@ -2816,6 +2816,8 @@ static const TypeIndexLookup c_TypeIndexLookup[] = {
     TIL("System.Reflection", "RuntimeFieldInfo", m_FieldInfo),
 
     TIL("System", "WeakReference", m_WeakReference),
+
+    TIL("System", "Guid", m_Guid),
 
     TIL("nanoFramework.UI", "Bitmap", m_Bitmap),
     TIL("nanoFramework.UI", "Font", m_Font),

src/CLR/Include/nanoCLR_Runtime.h

@@ -1552,6 +1552,8 @@ struct CLR_RT_WellKnownTypes
 
     CLR_RT_TypeDef_Index m_WeakReference;
 
+    CLR_RT_TypeDef_Index m_Guid;
+
     CLR_RT_TypeDef_Index m_SerializationHintsAttribute;
     CLR_RT_TypeDef_Index m_Bitmap;
     CLR_RT_TypeDef_Index m_Font;