Support exporting cut values

[trivialfis]

Deviating from the rest of the API functions, the value is returned as a json array interface. The choice is optional, but I want to start the discussion of this choice as we will return more complicated arrays in the future including CUDA arrays.

• Add functions to export cut values in C and Python.
• Move ellpackpage into src for accessing HistogramCuts.
• Tests.

Downstream projects can use the information to compress the tree model if needed.

[RAMitchell]

Is this a feature request?

[trivialfis]

No. I'm looking into enabling inference libraries to use the quantization result to compress the model.

[hcho3]

LGTM. (I only reviewed the Python portion)

Currently, Treelite performs unique operation to get the cut values from thresholds. This functionality will be useful, as we can skip the unique operation.

[trivialfis]

@hcho3 Please note that this PR doesn't work with approx, which changes cut value for each iteration

[trivialfis]

One will have to get the value for every iteration. Currently, this has to be done through some hacks.

[hcho3]

For now, Treelite can continue to scan the thresholds manually to compute the smallest integer width that will fit all thresholds.