Add support to String.Format

- This is required to use nanoFramework.CoreLibrary after nanoframework/CoreLibrary#12 is merged

Signed-off-by: José Simões <jose.simoes@eclo.solutions>

src/CLR/CorLib/corlib_native.cpp

@@ -694,6 +694,8 @@ static const CLR_RT_MethodHandler method_lookup[] =
     Library_corlib_native_System_String::ToLower___STRING,
     Library_corlib_native_System_String::ToUpper___STRING,
     Library_corlib_native_System_String::Trim___STRING,
+    NULL,
+    NULL,
     Library_corlib_native_System_String::Equals___STATIC__BOOLEAN__STRING__STRING,
     Library_corlib_native_System_String::op_Equality___STATIC__BOOLEAN__STRING__STRING,
     Library_corlib_native_System_String::op_Inequality___STATIC__BOOLEAN__STRING__STRING,
@@ -774,6 +776,7 @@ static const CLR_RT_MethodHandler method_lookup[] =
     NULL,
     NULL,
     NULL,
+    NULL,
     Library_corlib_native_System_Text_UTF8Decoder::Convert___VOID__SZARRAY_U1__I4__I4__SZARRAY_CHAR__I4__I4__BOOLEAN__BYREF_I4__BYREF_I4__BYREF_BOOLEAN,
     NULL,
     Library_corlib_native_System_Text_UTF8Encoding::GetBytes___SZARRAY_U1__STRING,
@@ -902,7 +905,7 @@ static const CLR_RT_MethodHandler method_lookup[] =
 const CLR_RT_NativeAssemblyData g_CLR_AssemblyNative_mscorlib =
 {
     "mscorlib", 
-    0x7207CF2D,
+    0x315C7581,
     method_lookup,
     { 1, 0, 0, 0 }
 };