[boost] Re: Decimal Library re-review begins today

`D` specifies `_Decimal64`, `H` specifies `_Decimal32`, and `DD` specifies `_Decimal128`. The `a` specifier for decimal floating-point types indicates quantum-preserving form and stands for "actual" (WG14's n1247). So, `a` might be a better fit for a {fmt}/<format> specifier.

a and A are already reserved for hexfloats. q and Q as in Quantum-Preserving or c and C for cohort-preserving are other options. Matt