Fix Issue 12556 - Add persistent stdio.File.byLine

https://issues.dlang.org/show_bug.cgi?id=12556

Add `stdio.File.byLineCopy`, which uses `ByLineImpl` internally.

* By default, lines are immutable. This can be changed by using a mutable `Char` type.
* `ByLineCopyImpl.front` is cached to avoid unnecessary allocations.
* The API follows `byLine` for consistency.
Ok, thanks. Looks like quite lot of `stdio` code has style notably different from i.e. `std.algorithm` :(