Similar to `after_unknown`, `before_sensitive` and `after_sensitive` are
values with similar structure to `before` and `after` which encode the
presence of sensitive values in a planned change. These should be used
to obscure sensitive values from human-readable output.
These values follow the same structure as the `before` and `after`
values, replacing sensitive values with `true`, and non-sensitive values
with `false`. Following the `after_unknown` precedent, we omit
non-sensitive `false` values for object attributes/map values, to make
serialization more compact.
One difference from `after_unknown` is that a sensitive complex value
(collection or structural type) is replaced with `true`. If the complex
value itself is sensitive, all of its contents should be obscured.
Alisdair McDiarmid