Followup to #3838 which is adding a benchmark - thanks @achabense!

• @AlexGuteniev suggested using basic_string::resize_and_overwrite, creating an _Ugly version for unconditional use internally. (We generally avoid adding totally novel secret machinery to Standard classes as it can become a maintenance burden, but simply having _Ugly names to access Future Technology is easy and common.)
• @AlexGuteniev also suggested investigating whether branchless codegen for assigning _Elem0 vs. _Elem1 would be superior. (Note: we cannot assume that their values are consecutive, they could be 'M' and 'E'.)
• I suggest investigating SIMD, as this would seem to be highly amenable to vectorization.

Regardless of what's investigated, I recommend profiling each change in isolation.