Remove PersistentCopy().

[gin-ahirsch]

First, we don't really need a shared_ptr from PersistentCopy(), since we're not sharing it with anyone yet. But then, we don't really need a pointer at all and we could just return by value and let call-sites move the value to the heap themselves if they need it there. At this point, the function body is trivial, so we don't really need the function.

[gin-ahirsch]

@nickbnf My filter-files branch (implementing #234, which I planned to open a PR for) does not apply cleanly on master without this change (or #236). What's the chance to merge this? I could adapt the filter-files branch, but if you want to pull this anyways I won't need to resolve conflicts twice.