Add test for Field.fromDouble.

[tixxit]

This adds a test for fromDouble - it uses the more brute-force version where we convert the Double to a BigDecimal, use that to extract a numerator/denominator, then use Ring.fromBigInt on both and divide.

[codecov-io]

Current coverage is 70.51% (diff: 100%)

Merging #191 into master will increase coverage by 2.61%

@@             master       #191   diff @@
==========================================
  Files            36         36          
  Lines           757        763     +6   
  Methods         703        709     +6   
  Messages          0          0          
  Branches         54         54          
==========================================
+ Hits            514        538    +24   
+ Misses          243        225    -18   
  Partials          0          0          

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[johnynek]

:+1: nice bump in coverage!

[non]

👍