Remove a level of indirection from internal std.xml.ElementParser code

[JackStouffer]

Removes an odd choice to keep the base string of a tag as a pointer to a string rather than the string itself.

I don't see an obvious reason why this was done, so I removed it for code simplicity and possible speed improvements.

[quickfur]

A pointer to a string might be necessary if the caller expects the callee to modify the string (as in, replace it with a different slice of the original data).

[quickfur]

In modern D we'd pass the string by ref, but this code may predate that, I'm not sure.

[quickfur]

But if the autotester passes, that probably means it was completely unnecessary and we can just merge this. :-D

[JackStouffer]

As best as I can tell, it looks like an attempt to avoid copying. Which is ridiculous as the string struct is tiny in reality and you end up having a pointer to a pointer for the actual data of the string, which is very bad for caches.

[quickfur]

I say we just merge this and see what explodes. We can clean up later. This code is badly in need of a major cleanup, new std.xml candidates notwithstanding.

[JackStouffer]

@quickfur Thanks. For more std.xml clean up please consider looking at #5378

[aG0aep6G]

This introduced a regression. See #5489.