refactor to reduce public api surface and more tightly scope names

This is a large-ish refactor to reduce the public API surface. Types
that are only needed within package scope were changed to reflect that.
Public names that might cause conflicts in the importing side have been
grouped into a new `OPA` namespace enum.

This change also begins documentation efforts using DocC.
Other changes are included to make the API more idiomatic.

* with/and
* store: read(from:) and write(to:)
* withCtx -> withContext
* init(fromJson:) -> init(jsonData:)
* defaultBuiltinRegistry -> static property
* BuiltinContext -> internal
* RegoValue type helpers -> package scope
* BuiltinsCache -> internal
* EvaluationContext -> internal
* unify error types under RegoError
* qualify Swift.Error
* Error -> Swift.Error
* StoreError -> RegoError
* rework BuiltinError in terms of RegoError, leaving adapters
* package visibility for ValueError
* remove EncodingError and incomplete encode methods
* prettyPrint(out:) -> prettyPrint(to:)
* prepareForEval->prepareForEvaluation
* RegoValue patch/merge, RegoEncodingError -> package visibility
* integrate functiondecl code from capabilities branch
* rename swift-rego -> swift-opa
* remove benchmarks - will return when further built out
* rename Bundle -> OPABundle to avoid name collisions
* remove unused Runtime module
* public -> internal CodingKeys
* update README.md
* remove unused StatementError
* OPABundle -> OPA.Bundle
* Manifest -> OPA.Manifest
* add documentation tags for some top-level types
* Engine -> OPA.Engine
* InMemStore -> OPA.InMemStore
* group tracing types under OPA.Trace
* BuiltinError -> enum

Signed-off-by: Oren Shomron <[email protected]>

Author: shomron

.gitignore

@@ -1,5 +1,5 @@
 .DS_Store
-/.build
+.build
 /Packages
 xcuserdata/
 DerivedData/