std.algorithm.copy could be better optimized

[TommiT]

std.algorithm.copy adheres to the specification of the C++ 
standard library function std::copy. That specification states 
that the source and target ranges may not overlap. Yet, all the 
major C++ standard libraries (libc++, libstdc++, Dinkum C++ 
Library) implement std::copy so that it becomes a memmove if the 
source and target element types are the same and the element type 
is std::is_trivially_copy_assignable. Thus, the current C++ 
specification seems to be too strict.

The current std.algorithm.copy implementation doesn't get 
optimized into a memmove when it would be safe to do so. It 
really should, because it's the fastest way to get the job done, 
and it allows the ranges to overlap. Also, the documentation 
should be changed to notify of this relaxation.

There's some benchmarks of memmove vs other methods:
http://nadeausoftware.com/articles/2012/05/c_c_tip_how_copy_memory_quickly