mangle C++ ::std and ::std::allocator correctly

On Linux, there are shortcut mangles for ::std and ::std::allocator which were not implemented.

Not done on Windows because C++ template mangling currently isn't done at all on Windows. Need to fix that with another PR.

Also wound up removing some tabs from the test files.
redundant control flow - just use

```d
if (exist(p->isTemplateInstance()->tempdecl))
{
    return true;
}
p = p->toParent();
```