The aim of this change is to make the `Doc` type more safe to use by approaching
the design via [Parse don't validate][[0]] approach.
The problem with the previous approach was that all field were `pub` and thus a
`Doc<Verified>` could easily be mutated and serialized. Granted, the code that
used the serialization would tend to verify the `Doc` first, however, this
approach *ensures* that only a verified `Doc` can be serialized. It also meant
that trying to add new data that would follow the parse approach would require
more generic parameters on top of the existing `PhantomData` parameter, i.e. we
need to do something like: `Doc<RawField, V> -> Doc<ValidField, V>`.
The new approach splits the type into two separate types: `RawDoc` and `Doc`.
The former is allowed to be mutated at will, and uses types that are less
strict. The latter is the valid type that can only be constructed by validating
a `RawDoc` (or the `initial` constructor). The `Doc` type's fields can then only
be accessed by read-only methods.
Solves the problems above by only allowing mutations to `RawDoc`, as well as,
new fields being added to `RawDoc` which are then validated via
`RawDoc::verified`.
[0]: https://lexi-lambda.github.io/blog/2019/11/05/parse-don-t-validate/
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