cleanup emplace code in Appender

This does a bit of fixup for #1529 .

The improvements done to emplace means it is "as safe" as what posblit gives us, and correct enough, that we can now use both it un-conditionally. This makes the code a *whole* lot simpler.

I also toned down some things which saved quite a few lines of code, and generally, just made everything cleaner.

It also contains the reg fix in #1683 for https://github.com/D-Programming-Language/phobos/pull/1529/files#r7536782