lib.fifo.AsyncFFSynchronizer: check input and output signal width

[rroohhh]

wasted more time than I would like to admit due to this :)

[codecov]

Codecov Report

Merging #517 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #517      +/-   ##
==========================================
+ Coverage   81.37%   81.38%   +0.01%     
==========================================
  Files          49       49              
  Lines        6410     6414       +4     
  Branches     1280     1282       +2     
==========================================
+ Hits         5216     5220       +4     
- Misses       1005     1006       +1     
+ Partials      189      188       -1     

Impacted Files	Coverage Δ
nmigen/lib/cdc.py	90.69% <100.00%> (+0.45%)	⬆️
nmigen/build/run.py	22.05% <0.00%> (ø)

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[whitequark]

A few nits:

• By convention, all error checking happens at the function entry.
• The error messages would be easier to read and more consistent with our other error messages if they used 1, not one.

[rroohhh]

async_edge is also not checked at the function entry, should that be changed aswell?

[whitequark]

Yup.

[whitequark]

Thanks!