Optimize <xstring> memmove function

[AZero13]

Optimized memmove

[fsb4000]

CI errors:

D:\build\out\inc\cstring(26,13): error: no member named 'memccpy' in the global namespace

using _CSTD memccpy;

      ~~~~~ ^

D:\build\out\inc\cstring(38,13): error: no member named 'strdup' in the global namespace; did you mean '_strdup'?

using _CSTD strdup;

      ~~~~~ ^~~~~~

            _strdup

C:\Program Files (x86)\Windows Kits\10\include\10.0.19041.0\ucrt\string.h(148,38): note: '_strdup' declared here

_ACRTIMP _CRTALLOCATOR char* __cdecl _strdup(

                                     ^

2 errors generated.

P.S. I doubt your PR will be accepted because Non-goal: Adding non-Standard extensions.
But I am not a maintainer of this repository so I could be wrong.
You will have a better chance that your PR will be accepted if you solve some existing issue.
Check out https://github.com/microsoft/STL/issues

[miscco]

I do not see any functional code change in product code. Please revert purely aesthetical changes

[AZero13]

I do not see any functional code change in product code. Please revert purely aesthetical changes

Done

[StephanTLavavej]

Thanks for this simplification, and congratulations on your first microsoft/STL commit! 🚀 😺 This will ship in VS 2022 17.0 Preview 4.

A note for the future: when pushing changes after reviewers have approved, it's a good idea to add a comment to the PR mentioning that you've done so, so the reviewers know to take another look. (Some changes that seem obviously desirable can be inconsistent with a coding convention, or can have surprising interactions with language complexity, etc. - which is why we're so careful about getting all changes to be seen by multiple reviewers.) Here, Casey and I had approved and moved this to Ready To Merge, and I wasn't looking closely for unannounced changes, so your last commit caught me by surprise. Fortunately Casey noticed and re-approved, so all was well (and I'm also fine with the change). This also happened before I mirrored the GitHub PR to the MSVC-internal repo, so no changes were dropped on the floor. I've now realized that we need to document these expectations better, so I'll add this to the Contribution Guidelines document that I need to write up.