allow column vectors as constants in coefficients

[avibryant]

cc @andrew-stripe for visibility

This makes a pair of individually conceptually small (if invasive) changes, but that combine to be quite significant:

• Coefficients, used in both Line and LogLine, now uses Constant nodes to hold the constant coefficient values, rather than directly using numeric values
• Column is switched from being treated as NonConstant to being treated as Constant

The overall effect here is that vector data (from using Fn) gets propagated through the graph rather than staying as leaf nodes, providing similar pre-computation benefits in the Fn case as in the direct case.

We aren't using Decimal, so two tests needed to be disabled. The Decimal logic will be brought back and folded into Constant.

[avibryant]

Seems like the lack of inlining is making tests timeout. Will keep this PR unmerged until the follow-ups have resolved that.

[codecov-io]

Codecov Report

Merging #444 into 0.3-dev will decrease coverage by 3.65%. The diff coverage is 79.41%.

@@             Coverage Diff             @@
##           0.3-dev     #444      +/-   ##
===========================================
- Coverage    54.67%   51.02%   -3.66%     
===========================================
  Files           73       74       +1     
  Lines         2758     2834      +76     
  Branches       178      180       +2     
===========================================
- Hits          1508     1446      -62     
- Misses        1250     1388     +138

Impacted Files	Coverage Δ
...main/scala/com/stripe/rainier/compute/Bounds.scala	89.79% <ø> (-0.21%)	:arrow_down:
...a/com/stripe/rainier/bench/stan/GLMMPoisson2.scala	0% <0%> (ø)	:arrow_up:
...ain/scala/com/stripe/rainier/compute/RealViz.scala	0% <0%> (ø)	:arrow_up:
...in/scala/com/stripe/rainier/compute/Gradient.scala	93.1% <100%> (-0.12%)	:arrow_down:
...cala/com/stripe/rainier/compute/Coefficients.scala	88.88% <100%> (-6.28%)	:arrow_down:
.../com/stripe/rainier/ir/OutputMethodGenerator.scala	100% <100%> (ø)	:arrow_up:
...n/scala/com/stripe/rainier/compute/Evaluator.scala	63.63% <100%> (+2.76%)	:arrow_up:
...ain/scala/com/stripe/rainier/ir/DataFunction.scala	100% <100%> (ø)	:arrow_up:
...in/scala/com/stripe/rainier/compute/Compiler.scala	100% <100%> (ø)	:arrow_up:
.../scala/com/stripe/rainier/compute/LogLineOps.scala	87.5% <100%> (-12.5%)	:arrow_down:
... and 20 more

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