Imports API

[travisbrown]

This PR includes a few API changes for the imports module that I thought we could get in before 0.2.0, primarily with the goal of minimizing our binary compatibility obligations in future releases.

I've promoted a couple of types that were previously defined in package-private objects but were also exposed in public method signatures in other places, and which felt like they should be in the public API (given usage in tests, etc.):

• ResolveImportsVisitor.ImportContext is now ImportContext.
• Caching.ImportsCache is now ImportCache (I've depluralized it for consistency).

I've made a few things that were previously case classes (like ResolveImportsVisitor and NoopImportCache) not case classes, since they don't really need to be, and since case classes with type class constraints are kind of weird (e.g. it's not really clear what equality should mean for two ResolveImportsVisitor values with different Fs or different clients or Sync instances).

I've added a new ResolveImports object that provides a non-enrichment-method-based version of the top-level API, providing two methods (essentially Expr => F[Expr] and (ImportCache, Expr) => F[Expr]). This allows us to avoid using ResolveImportsVisitor directly in tests, which feels a little better to me. I've added an Ops value class for the resolveImports enrichment method there (instead of an implicit class in the package object), since this keeps similar concerns in the same place, and is closer to the pattern used by Cats, etc.

I've made ResolveImportsVisitor and a couple of other things properly private instead of package-private, since this frees them up from binary compatibility concerns (since Scala's access modifiers are a terrible mess and package privacy is just public from the JVM's point of view).

I've also changed CORSComplianceCheck to CorsComplianceCheck for consistency with most other acronym-including names in the project (like JsonConverter, YamlConverter, etc.; I also went ahead and changed a couple of instances of CBORX to CborX here as well).

[travisbrown]

@TimWSpence I'm thinking of two additional changes:

• Rename the new ResolveImports (which was an abbreviation of ResolveImportsVisitor) to Resolver, so we have org.dhallj.imports.Resolver following the pattern of org.dhallj.yaml.YamlConverter, org.dhallj.parser.DhallParser, etc.
• Move the enrichment method conversion to org.dhallj.imports.syntax._, following the rule from Cats and other Scala modules in this package that enrichment method imports are always flagged by a syntax package. (At some point we could also add a Cats-style syntax.all._ import.)

What do you think?

[TimWSpence]

LGTM!