Rather than casting the FixedPoint to double, format the FixedPoint
directly. This avoids using floating point instruction, which in
turn enables this to be used even in the kernel.
Because AK/Concepts.h includes AK/Forward.h and concepts cannot be
forward declared, slightly losen the FixedPoint template arguments
so that we can forward declare it in AK/Forward.h
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.
