Improve Record.bind and Detect Records with unstable elements (backport #2829) (#2831)

* Add Aggregate.elementsIterator and micro-optimize

elementsIterator provides a more efficient API for iterating on the
elements of Aggregates. It is especially useful for Records where
getElements returns a Seq and thus eagerly constructs a new
datastructure which may then just be iterated on anyway.

This new elementsIterator API is then used throughout the codebase where
it makes sense.

Also change Vec.getElements to just return the underlying self instead
of constructing a new Seq.

(cherry picked from commit defa440b)

# Conflicts:
#	core/src/main/scala/chisel3/Aggregate.scala
#	core/src/main/scala/chisel3/Module.scala
#	core/src/main/scala/chisel3/experimental/Trace.scala

* Move Aggregate.bind inline into Record.bind

Vec overrides bind and does not call the version in Aggregate so the
version in Aggregate is misleading in that its only ever used by
Records. Now there is no version in Aggregate and the actual
functionality and use is more clear.

(cherry picked from commit b054c30b)

# Conflicts:
#	core/src/main/scala/chisel3/Aggregate.scala

* Extract and optimize duplicate checking Record.bind

This replaces an immutable.Map with a single mutable.HashSet and saves
the allocation of # elements Seqs.

(cherry picked from commit 832ea52b)

# Conflicts:
#	core/src/main/scala/chisel3/Aggregate.scala

* Add check for Records that define def elements

(cherry picked from commit a4f22341

)

* Resolve backport conflicts

* Make elementsIterator final and package private

* Waive false MiMa failure

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