walking avoids overflow on cyclic types

walking ignores private fields

use generics for type filters and `Appender`

rename `WalkFuncPtr` → `WalkPtrFunc`

add `WalkPtr`, `WalkFuncPtr`; remove `DerefPtr`

`DeepFields` for struct flattening; other misc

add `IfaceFilterFor`, `ShallowIfaceFilterFor`

split `IfaceFilter` and `ShallowIfaceFilter`

`IfaceFilter` visits either self or descendants, not both

filters return flagsets; descendant walking is optional