The folded name logic (despite all attempts to optimize it)
was fundamentally an O(n) operation where every field in a struct
needed to be linearly scanned in order to find a match.
This made unmashaling of unknown fields always O(n).
Instead of optimizing the comparison for each field,
make it such that we can look up a name in O(1).
We accomplish this by maintaining a map keyed by pre-folded names,
which we can pre-calculate when processing the struct type.
Using a stack-allocated buffer, we can fold the input name and
look up its presence in the map.
Also, instead of mapping from names to indexes,
map directly to a pointer to the field information.
The memory cost of this is the same and avoids an extra slice index.