Added :<>=, :<=, :>=, :#=

[azidar]

In general, Chisel's connection operators are very broken; their semantics change depending on import Chisel._ vs import chisel3._. Even chisel3._'s semantics are not clean; you cannot reason about what will be connected to what unless you know whether the things you are connecting are Wires or IOs. Part of this stems from the fact that we got rid of relative-flippedness for declaring things, instead relying on using Input/Output.

This PR defines four new operators which have very clean semantics which depend solely on the hardware type of the things you are connecting. Using them, we plan to migrate all existing operators semantics to compositions of them, but for now we define them independently of Chisel/chisel3, and thus they have more verbose syntax to make them compatible with both imports.

As we continue, we may rename them etc., but their semantics are composable which is an important property. In fact, there are only actually two operators which, when composed in different ways, can implement the semantics of other kinds of operators: :<= and :>=. While these operators may not be the most common ones to use, understanding them will help users reason about how all the connection operators work.

I've written a lot of ScalaDoc to describe their semantics. Future work can write more explanative material (mdoc etc.) to flesh out them in more detail.

Contributor Checklist

• [x] 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?
• [ ] Did you add appropriate documentation in docs/src?
• [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

No impact on current code. Adds new operators and documentation.

Desired Merge Strategy

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

Release Notes

• Added new operators :<>=, :<=, :>=, :#=
• Added new Scaladoc to :=, <>

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?
• [ ] Did you check whether all relevant Contributor checkboxes have been checked?
• [ ] 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.