Refactor REPL to use rego package for partial eval
Refactor REPL to use rego package for partial eval

Previously the REPL was using topdown directly because the rego package
did not expose an interface to obtain raw partial evaluation results.

Signed-off-by: Torin Sandall <933af5bec0bc58056526d55ca6c7b45fcf14cc48@gmail.com>

tsandall/opa,open-policy-agent/opa,tsandall/opa,open-policy-agent/opa,open-policy-agent/opa,open-policy-agent/opa,tsandall/opa,tsandall/opa,tsandall/opa,tsandall/opa,open-policy-agent/opa,open-policy-agent/opa