Add support for explicit names in properties

[jeanthom]

This PR implements explicit naming of properties (Assert/Assume/Cover) as asked in #744. The name is specified through the use of a new optional parameter "name".

With the following code example:

from amaranth import *
from amaranth.asserts import *
from amaranth.back import rtlil

m = Module()
a = Signal()
m.d.sync += Assert(a != 0, name="foobar")

print(rtlil.convert(m, ports=(a,)))

The $assert cell picks up the proper name:

[...]
  cell $assert \foobar
    connect \A $assert$check
    connect \EN $assert$en
  end
[...]

Fixes #744.

[whitequark]

See comments, LGTM otherwise.

[codecov-commenter]

Codecov Report

Merging #747 (d15d9ee) into main (666ee27) will decrease coverage by 0.08%.
The diff coverage is 44.44%.

@@            Coverage Diff             @@
##             main     #747      +/-   ##
==========================================
- Coverage   81.20%   81.13%   -0.08%     
==========================================
  Files          49       49              
  Lines        6487     6492       +5     
  Branches     1557     1559       +2     
==========================================
- Hits         5268     5267       -1     
- Misses       1027     1032       +5     
- Partials      192      193       +1     

Impacted Files	Coverage Δ
amaranth/back/rtlil.py	78.43% <ø> (ø)
amaranth/hdl/ast.py	89.18% <33.33%> (-0.38%)	⬇️
amaranth/hdl/xfrm.py	96.23% <66.66%> (ø)
amaranth/_toolchain/cxx.py	95.23% <0.00%> (-4.77%)	⬇️
amaranth/build/run.py	21.58% <0.00%> (ø)

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[whitequark]

LGTM

[whitequark]

Thank you!