This isn't a complete conversion to ErrorOr<void>, but a good chunk.
The end goal here is to propagate buffer allocation failures to the
caller, and allow the use of TRY() with formatting functions.
When I added this code in 1472f6d, I forgot to add tests for it. That's
why I didn't realize that the values were appended to the wrong
FormatBuilder object, so an empty string was returned instead of the
expected "nan"/"inf". This made debugging some FPU issues with the
ScummVM port significantly more difficult.
The state of the formatter for the previous element should be thrown
away for each iteration. This showed up when trying to format a
Vector<String>, since Formatter<StringView> was unhappy about some state
that gets set when it's called. Add a test for Formatter<Vector>.
