This is a particularly hot function, so we operate directly on encodings
rather than the more straightforward implementation of calling
`indexToKey`.

I measured this as 1.05 ± 0.04 times faster than the previous commit
with a ReleaseFast build against hello world (which includes std.debug
and formatted printing).

I also profiled the function and found that zigTypeTag() went from being
a major caller of `indexToKey` to being completely insignificant due to
being so fast.