Change how `AttributeSet::Builder` receives its defaults

There are two concerns which are both being combined into one here, but
both have the same goal. There are certain attributes which we want to
always consider initialized. Previously, they were handled separately.
The primary key (which is assumed to be backed by a database column)
needs to be initialized, because there is a ton of code in Active Record
that assumes `foo.id` will never raise. Additionally, we want attributes
which aren't backed by a database column to always be initialized, since
we would never receive a database value for them.

Ultimately these two concerns can be combined into one. The old
implementation hid a lot of inherent complexity, and is hard to optimize
from the outside. We can simplify things significantly by just passing
in a hash.

This has slightly different semantics from the old behavior, in that
`Foo.select(:bar).first.id` will return the default value for the
primary key, rather than `nil` unconditionally -- however, the default
value is always `nil` in practice.

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