Improve unboxing of strict fields
Note [Recursive unboxing] ~~~~~~~~~~~~~~~~~~~~~~~~~ Be careful not to try to unbox this! data T = MkT !T Int But it's the *argument* type that matters. This is fine: data S = MkS S !Int because Int is non-recursive. Before this patch, we were only doing the unboxing if the *parent* data type was non-recursive (eg that meant S was not unboxed), but that is over-conservative. This showed up with indexed data types (thanks to Roman for finding it) because indexed data types are conservatively regarded as always recursive.