Add :<>=, which is a chisel3 API for FIRRTL.<= (to aid in migration from Chisel.:= and Chisel.<>)

[azidar]

To migrate code from Chisel._ to chisel3._, the bulk connect operators are very different, which requires a significant rewrite of every bulk connection.

However, the old connect operator was actually well defined, and a good operator. Hence, we want to let import chisel3._ users have the old connect operator back, but with a new syntax.

NOTE: this new operator does not have the exact same semantics as Chisel.:= or Chisel.<> because the chiselTypes must be the same on both sides.

Contributor Checklist

• [ ] Did you add Scaladoc to every public function/method?
• [x] Did you add at least one test demonstrating the PR?
• [x] Did you delete any extraneous printlns/debugging code?
• [x] Did you specify the type of improvement?
• [N/A] Did you add appropriate documentation in docs/src? Will update this when other connect ops :<= and :>= are added.
• [x] Did you state the API impact?
• [x] Did you specify the code generation impact?
• [x] Did you request a desired merge strategy?
• [x] Did you add text to be included in the Release Notes for this change?

Type of Improvement

• new feature/API

API Impact

Backend Code Generation Impact

No change - it is a new API.

Desired Merge Strategy

• Squash: The PR will be squashed and merged (choose this if you have no preference.

Release Notes

Added the :<>= operator, semantically equivalent to Chisel.:= but available in import chisel3._.

Reviewer Checklist (only modified by reviewer)

• [ ] Did you add the appropriate labels?
• [ ] Did you mark the proper milestone (Bug fix: 3.4.x, [small] API extension: 3.5.x, API modification or big change: 3.6.0)?
• [ ] Did you review?
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• [ ] Did you do one of the following when ready to merge:
  • [ ] Squash: You/ the contributor Enable auto-merge (squash), clean up the commit message, and label with Please Merge.
  • [ ] Merge: Ensure that contributor has cleaned up their commit history, then merge with Create a merge commit.

[mwachs5]

Is this FIRRTL.<- ? I didn't this firrtl has a :=?

[mwachs5]

Implements a piece of #2643

[azidar]

chisel3.:<>= is FIRRTL.<=, which is also Chisel.:=.

[jackkoenig]

chisel3.:<>= is FIRRTL.<=, which is also Chisel.:=.

Chisel.:= is actually FIRRTL.<- when used on an aggregate. In fact, := and <> in import Chisel._ are exactly the same 😬 (Scastie)

[mwachs5]

This PR title is misleading because the chisel3.:<>= operator doesn't have the same semantics as Chisel.:=. I show this by copying over a test from CompatibilitySpec which uses Chisel.:= to connect missing bundles fields which fails using chisel3.:<>= : https://github.com/chipsalliance/chisel3/pull/2641/commits/f6e3d217c1e7d07f38ff4f985dc1729c6dfcaba9

I'm not saying this new operator is not useful, it's just misleading to characterize it as the same semantics as Chisel.:=

[azidar]

Subsumed by #2713