P1233 Shift-by-negative in shift_left and shift_right

[jensmaurer]

P1233R1 Shift-by-negative in shift_left and shift_right (Ashley Hedberg, Matt Calabrese, Bryce Adelstein Lelbach)

https://issues.isocpp.org/show_bug.cgi?id=541

[mclow]

Superseded by P1243 Rangify New Algorithms #152.