This would previously fail to compile when included in files that do not
include FileUtil.h due to lack of a type declaration.
This moves the constructor and destructor into the cpp file in order to
satisfy the requirements of unique_ptr construction and deletion. That is,
unique_ptr requires a concrete type at the point of construction and
destruction. If the constructor or destructor is left in the header, then
at the point of construction or destruction, IOFile will still be
considered an incomplete type, as unique_ptr's deleter will still only be
able to see the forward declaration, which it can't use.
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