Enhance views to [sometimes] support dynamic indexing and implement FlatIO (backport #2476) (#2479)

* Capture 1:1 mappings of Aggregates inside of views

This is implemented by including any corresponding Aggregates from the
DataView.mapping in the AggregateViewBinding.childMap (which is now of
type Map[Data, Data]).

This enables dynamically indexing Vecs that are themselves elements of
larger Aggregates in views when the corresponding element of the view is
a Vec of the same type. It also increases the number of cases where a
single Target can represent part of a view.

(cherry picked from commit 1f6b1ca1)

* Add FlatIO API for creating ports from Bundles without a prefix

(cherry picked from commit 772a3a1f)

* [docs] Add FlatIO to the general cookbook

(cherry picked from commit b4159641

)

Co-authored-by: Jack Koenig <koenig@sifive.com>