<string>: Consider retuning the Small String Optimization

Currently, our basic_string implementation can store up to 15 narrow characters, or 7 wide characters, before dynamically allocating memory:

https://github.com/microsoft/STL/blob/58bb49d63d92e7a0346a05af29816aeea6b4cf0f/stl/inc/xstring#L2134-L2135
https://github.com/microsoft/STL/blob/58bb49d63d92e7a0346a05af29816aeea6b4cf0f/stl/inc/xstring#L2185-L2196

We should consider retuning this optimization, given platform evolution over the last 20 years.

@BillyONeal commented: "Note that libc++'s implementation is a full pointer narrower than ours and stores up to 22 characters instead of 15; optimally small."

Related note: std::function's Small Functor Optimization was retuned before VS 2015 froze binary compatibility, using the heuristic that a function object storing a std::string should be considered small (regardless of architecture or debug mode). We don't necessarily have to retune std::function again, but we should at least check that the heuristic is still satisfied.

Also tracked by Microsoft-internal VSO-154236 / AB#154236.

vNext note: Resolving this issue will require breaking binary compatibility. We won't be able to accept pull requests for this issue until the vNext branch is available. See #169 for more information.

Start with the referenced implementation in stl/inc/xstring, especially the cited basic_string layout and capacity logic. Compare its small-string assumptions with libc++ and inspect the std::function small-functor heuristic described in the issue. This work must wait for the vNext branch and is complete only when the optimization is retuned while accounting for the required binary-compatibility break.