wrap BigDecimal in Decimal

[avibryant]

This is a first step towards eliminating BigDecimal: it wraps all uses of BigDecimal in our own Decimal object.

[avibryant]

cc @sritchie

[avibryant]

It's failing on compiling rainier-cats right now, I'll fix that in a bit

[codecov-io]

Codecov Report

Merging #431 into 0.3-dev will increase coverage by 0.16%. The diff coverage is 90.27%.

@@             Coverage Diff             @@
##           0.3-dev     #431      +/-   ##
===========================================
+ Coverage    58.58%   58.74%   +0.16%     
===========================================
  Files           67       69       +2     
  Lines         2622     2642      +20     
  Branches       144      153       +9     
===========================================
+ Hits          1536     1552      +16     
- Misses        1086     1090       +4

Impacted Files	Coverage Δ
...n/scala/com/stripe/rainier/compute/Evaluator.scala	61.53% <ø> (ø)	:arrow_up:
...om/stripe/rainier/bench/CategoricalBenchmark.scala	0% <0%> (ø)
.../scala/com/stripe/rainier/compute/Translator.scala	95% <100%> (ø)	:arrow_up:
...main/scala/com/stripe/rainier/compute/ToReal.scala	73.33% <100%> (ø)	:arrow_up:
...ain/scala/com/stripe/rainier/compute/LineOps.scala	94.59% <100%> (ø)	:arrow_up:
...in/scala/com/stripe/rainier/compute/Gradient.scala	93.22% <100%> (ø)	:arrow_up:
...c/main/scala/com/stripe/rainier/cats/package.scala	85.1% <100%> (ø)	:arrow_up:
...cala/com/stripe/rainier/compute/Coefficients.scala	95.16% <100%> (ø)	:arrow_up:
.../scala/com/stripe/rainier/compute/LogLineOps.scala	100% <100%> (ø)	:arrow_up:
...main/scala/com/stripe/rainier/compute/Bounds.scala	96% <100%> (ø)	:arrow_up:
... and 7 more

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