Code writer/visitors: Remove unnecessary implicit conversions

For example, this:
```csharp
int i = 5;
Expression<Func<long>> expr = () => (long)i;
Console.WriteLine(expr.ToString("C#"));
```
produces this:
```none
() => (long)i
```
when the following:
```csharp
() => i
```
is also valid. Even though the precise return type is now elided, but having the conversion may obscure other parts of the structure of the tree.