Add declass module to preprocess a style's paint classes

[anandthakker]

Prepares for removal of paint classes from GL JS by providing declass as a migration path for projects relying on them. map.setClasses(classes) will become map.setStyle(declass(myBaseStyleJSON, classes)).

Couple notes:

• Currently I have declass leaving the paint.foo properties on layers, even after applying the foo class, but I'm not sure whether that's a good idea.
• Based on my admittedly brief skim of GL JS paint class code, I believe there's no provision for a paint class to unset a property, and so that's the case here as well. One implication of this is that users may need to hang on to their original style JSON if they want to flip back and forth between style-with-no-class-applied and style-with-a-class-applied -- or else they'll need to explicitly declare in the base paint definition the value for any property that's defined in a paint.blah definition

[jfirebaugh]

• Currently I have declass leaving the paint.foo properties on layers, even after applying the foo class, but I'm not sure whether that's a good idea.

It should remove them -- the output should include only plain paint properties.

• Based on my admittedly brief skim of GL JS paint class code, I believe there's no provision for a paint class to unset a property, and so that's the case here as well.

Correct, they are additive only.

[anandthakker]

It should remove them -- the output should include only plain paint properties

@jfirebaugh removed.