Implement compressed Namespace (backport #2856) (#2860)

* Implement compressed Namespace (#2856)

The namespace disambiguates requests for the same name with _<idx>.
Rather than storing every disambiguated name in the underlying HashMap,
it now only stores the base along with the "next available" index. This
makes the logic for checking if a name is already contained in the
namespace slightly more sophisticated because users can name things in a
way that will collide with disambiguated names from a common substring.

For example, in naming the sequence "foo", "foo", "foo_1", the 2nd "foo"
takes the name "foo_1" so the following "foo_1" gets disambiguated to
"foo_1_1". But since we compressed that original "foo_1" into the same
HashMap entry as just "foo", we have to do a form of "prefix checking"
whenever naming something that ends in "_<idx>".

In practice, the saved memory allocations more than make up for the more
complicated logic to disambiguate names because the common case is still
fast.

(cherry picked from commit 1654d87a02ca799bf12805a611a91e7524d49843)

# Conflicts:
#	core/src/main/scala/chisel3/internal/Builder.scala

* Resolve backport conflicts

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

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