Add src parameter to WireVector, Output, and Register

[timsherwood]

Summary

• Adds an optional src parameter to WireVector, Output, and Register constructors, allowing wire connections at construction time.
• For WireVector and Output, src=x is equivalent to wire <<= x.
• For Register, src=x is equivalent to reg.next <<= x.

Example

a = pyrtl.Input(name="a", bitwidth=8)
b = pyrtl.Input(name="b", bitwidth=8)
begin = pyrtl.Input(name="begin", bitwidth=1)

x, done = gcd(a, b, begin)

x = pyrtl.Output(name="x", src=x)
done = pyrtl.Output(name="done", src=done)

Notes

• Input and Const do not accept src (raises TypeError).
• Self-referencing register patterns like reg = Register(src=reg + 1) are not possible; use reg.next <<= reg + 1 for those cases.
• The block parameter is moved to the last position in all affected constructors for consistency.

Test plan

• All 1382 existing tests pass
• 11 new tests covering WireVector, Output, and Register src usage, bitwidth inference, coercion, and negative cases

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.8%. Comparing base (d0021d4) to head (5c9666c).

Additional details and impacted files

@@              Coverage Diff              @@
##           development    #512     +/-   ##
=============================================
+ Coverage         93.8%   93.8%   +0.1%     
=============================================
  Files               30      30             
  Lines             7174    7178      +4     
=============================================
+ Hits              6724    6729      +5     
+ Misses             450     449      -1     

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[fdxmw]

Thanks for putting this together! Some questions/comments:

• I think it's better to leave the block argument where it was, currently this change is not backwards-compatible. Users depending on positional argument order will likely have problems after this change.
• What do you think about renaming src to source? IMO three vowels are a small cost for dictionary-safety :)
• What about naming Register's constructor argument next, to match Register.next?
• What do you think about making these new constructor arguments keyword-only? IMO that would improve readability, I think it's quite hard to figure out what this code does, without consulting the documentation:

  wire = WireVector(2, "three", 3)

• Along those lines, this change makes it easier to write inefficient code, like the example above, which should just be Const(3, name="three"), rather than a Const connected to a WireVector. Here's another:

  wire = WireVector(src=other)

  It is a little annoying that we can't skip construction of these useless objects. Should we emit a warning when we can detect cases like these?