Improve Instance driver-driver conflict error message

[awygle]

This PR attempts to address issues #405 , #398 , and #320 by improving error messages for driver-driver conflicts involving Instances.

I took my best stab at a better message, but am totally open to changing it.

I am almost sure an Instance conflict is the only case in which this code path is hit (I called out "input pin" because while it's the same issue under the hood it looks different to the user), but please do review closely.

No rush on this - not sure if it makes sense to go into 0.3 or not.

[codecov[bot]]

Codecov Report

Merging #596 (ad07fcf) into master (f7c2b94) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #596   +/-   ##
=======================================
  Coverage   81.50%   81.50%           
=======================================
  Files          49       49           
  Lines        6461     6462    +1     
  Branches     1287     1288    +1     
=======================================
+ Hits         5266     5267    +1     
  Misses       1007     1007           
  Partials      188      188           

Impacted Files	Coverage Δ
nmigen/hdl/ir.py	95.48% <100.00%> (+0.01%)	:arrow_up:

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[codecov-commenter]

Codecov Report

Merging #596 (1699ae4) into main (2adbe59) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #596   +/-   ##
=======================================
  Coverage   81.53%   81.53%           
=======================================
  Files          49       49           
  Lines        6468     6468           
  Branches     1531     1532    +1     
=======================================
  Hits         5274     5274           
+ Misses       1003     1002    -1     
- Partials      191      192    +1     

Impacted Files	Coverage Δ
amaranth/hdl/ir.py	95.50% <100.00%> (+0.01%)	:arrow_up:
amaranth/sim/pysim.py	93.17% <0.00%> (-0.11%)	:arrow_down:
amaranth/lib/fifo.py	94.08% <0.00%> (ø)
amaranth/build/run.py	22.05% <0.00%> (ø)
amaranth/sim/_pycoro.py	80.00% <0.00%> (+0.23%)	:arrow_up:
amaranth/sim/core.py	83.67% <0.00%> (+0.34%)	:arrow_up:

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[whitequark]

Obsoleted by #1104.