Mapping for layers with functions

[Mattriks]

• I've added and/or updated the unit tests
• I've run the regression tests
• I've built the docs and confirmed these changes don't cause new errors

This PR:

• fixes an issue identified on discourse:
  Point 2. layer(Function, ..., order=2) doesn't work.

Example:

using RDatasets
iris = dataset("datasets", "iris")

fs = [ x->0.3x+1, x->0.3x+1.1 ]
cs = ["versicolor", "virginica"]
plot(iris,
  layer(x=:SepalLength, y=:SepalWidth, color=:Species),
  layer(fs, 4, 8, color=cs, order=2),
)

[codecov-io]

Codecov Report

Merging #1420 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1420   +/-   ##
=======================================
  Coverage   89.57%   89.57%           
=======================================
  Files          39       39           
  Lines        4288     4288           
=======================================
  Hits         3841     3841           
  Misses        447      447           

Impacted Files	Coverage Δ
src/poetry.jl	90.74% <100.00%> (ø)

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