Add rotate by constant

[Ravenslofty]

I think I got the signs right, anyway.

Closes #339.

[whitequark]

Please add some tests. In this case, some assertRepr tests in test_hdl_ast.py, and a functional test in test_sim.py. In the latter, if you write the literals in binary, it'll be easy to see if you got the signs right.

[codecov]

Codecov Report

Merging #352 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #352      +/-   ##
==========================================
+ Coverage   82.69%   82.72%   +0.02%     
==========================================
  Files          35       35              
  Lines        5919     5940      +21     
  Branches     1201     1206       +5     
==========================================
+ Hits         4895     4914      +19     
- Misses        863      864       +1     
- Partials      161      162       +1     

Impacted Files	Coverage Δ
nmigen/hdl/ast.py	88.71% <100.00%> (+0.18%)	⬆️
nmigen/hdl/rec.py	97.51% <0.00%> (-1.19%)	⬇️
nmigen/build/run.py	31.25% <0.00%> (ø)
nmigen/build/plat.py	28.15% <0.00%> (ø)

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[whitequark]

Please add tests for rotates by non-1, and in particular for rotates by values larger than the value size, since currently that would be an error.

Also, if you think negative rotates should be supported, tests for those, too. If you think they should not be, then add raise SyntaxError if they are encountered. (Personally I am fine with either option.)

[whitequark]

Should be good after one more round of changes.