Issue 648 - Expand template mixins in ddoc

http://d.puremagic.com/issues/show_bug.cgi?id=648

This will expand template mixin member documentation into the scope of the aggregate or module the mixin was used in. IOW:

```D
mixin template Mixin()
{
    /// documented field
    int field;
}

/// class A
class A
{
    /// undocumented statement
    mixin Mixin!();
}
```

Will expand into ddoc output:

```D
/// class A
class A
{
    /// documented field
    int field;
}
```

The expansion works recursively, as template mixin statements might be embedded inside of other template mixins.

This is only an implementation for `template mixin` statements, and not for `string mixins` (or `CompileStatement`'s as they're known internally).