Convert functions to expressions; add interpolation color spaces

[jfirebaugh]

Convert functions to expressions for evaluation (reverses #5379). Adds support for interpolate-hcl and interpolate-lab expression operators to replace the colorSpace function property (fixes #5326). Switches to shortest-path interpolation for HCL (fixes #5811).

Evaluation of converted expressions is slightly faster:
https://bl.ocks.org/anonymous/raw/ea7442aff28940abb9a358501aa9067b/

However, conversion+creation is slower. I may try to reimplement conversion to directly construct expression instances, rather than converting to a JSON representation.
https://bl.ocks.org/anonymous/raw/63b5d4b704f55e8e50dd5871297a1490/

What's still not working currently is the default property for functions. @anandthakker, I noticed the function tests for this were skipped. It looks like the skips were added as part of #4777. Do you recall if we ever came up with a plan for how to convert defaults into expression syntax?

[anandthakker]

Do you recall if we ever came up with a plan for how to convert defaults into expression syntax?

I think now that we're handling spec-defined defaults outside of expression evaluation (i.e. in a catch block in the error-handling wrapper function), it would probably be simplest to handle user-defined defaults that way, too. Including functions' defaulting semantics in the converted expression would be possible, but undesirably complicated.

[jfirebaugh]

Shelving this for now. Conversion to JSON intermediate format isn't fast enough, and it turned out to be difficult and ugly to implement direct conversion to Expression subclasses.