Update to use generics when GO 1.18 drops

[trypsynth]

There's a lot of interface{} in this code. When Go 1.18 gets released later this month, LIT should probably be updated to take advantage (interface{} can be extremely unsafe, because it's basically equivalent to a void* pointer in C because you can cast it to literally any other type and Go will just happily take it and go "Yay, this is apparently this type now, so let's make it into that").