15.2.2.2 (and more widely) Unconventional spacing around colons

Currently we have:

``` csharp
abstract class B: A
```

I think it's more conventional to include a space after the type name:

``` csharp
abstract class B : A
```

We've talked about this before for things like bracing, where the standard is slightly unconventional but saves space - in this case I'd very much doubt that it will make any lines too long. I want to make the examples as unjarring as possible.

If we agree with the proposal (adding the space) I'll go through the standard and identify which examples need to be changed.