[2018-06 LWG Motion 19] P0769R2 Add shift to <algorithm>

[jensmaurer]

[timsong-cpp]

This one needs some fixes:

• shift_left's Returns: uses the expression first + (last - first) - n, this should be reparenthesized to first + (last - first - n) or first + ((last - first) - n). There's no library wording defining a - n for forward iterators.
• We don't have a CamelCased BidirectionalIterator requirement, so shift_right's "the BidirectionalIterator requirements" (in both the Requires and Effects paragraphs) should be "the requirements of a bidirectional iterator", with a cross-reference to [bidirectional.iterators].

[AlisdairM]

We use iterator arithmetic, even for forward iterators, where the semantics are clear. Every so often there are attempts to clean it up, but if precision comes at the cost of too much clarity, the shorthand usually wins.