Undo move of Cat from chisel3.util to chisel3 (backport #3110) (#3111)

* Undo move of Cat from chisel3.util to chisel3 (#3110)

It is a very painful change for users with marginal benefit. We plan to
change the package to chisel in the not-so-distant future--that's a much
better opportunity to make this change.

(cherry picked from commit 036b1e90

)

* Resolve backport conflicts

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