Output Iterators should not be provided as only `begin` and no `end`

[SSoelvsten]

Taking a look at std::copy, it is quite odd that we use a first and last iterator. In fact, we don't allow one to use std::back_inserter(to_vector) as is desired.

• [ ] Change output iterators to be provided as a singular value. Most code can be reused as-is; the tests primarily need updating.
• [ ] Move the old versions into adiar/deprecated.h (as a mere alias for the new one ones)