We will probably readd these as an opaque object passed down to
conversions that lets the caller get access to more info (like
a negotiated serializer).
reflect.Call is fairly expensive, performing 8 allocations and having to
set up a call stack. Using a fairly straightforward to generate switch
statement, we can bypass that early in conversion (as long as the
function takes responsibility for invocation). We may also be able to
avoid an allocation for the conversion scope, but not positive yet.
```
benchmark old ns/op new ns/op delta
BenchmarkPodConversion-8 14713 12173 -17.26%
benchmark old allocs new allocs delta
BenchmarkPodConversion-8 80 72 -10.00%
benchmark old bytes new bytes delta
BenchmarkPodConversion-8 9133 8712 -4.61%
```
