Added support for defining functions which have a tuple as a generic parameter. (#285)

Previously defining this would work:

```csharp
Linker.DefineFunction<int, (int, int)>("module", "name", (int a) => (a, a));
```

But doing this would not work:

```csharp
void DefineWrapper<TIn, TOut>(Func<TIn, TOut> func)
{
    Linker.DefineFunction<TIn, TOut>("module", "name", func);
}

DefineWrapper<int, (int, int)>((int a) => (a, a));
```

Even though they're ultimately doing the same thing, just with one more level of generic typing.

Author: martindevans