Interpreter: Export 'InterpretEnv' as a newtype
Interpreter: Export 'InterpretEnv' as a newtype

The REPL will need to hang onto an 'InterpretEnv', and I cringe to
export 'InterpretEnv' from 'SAWScript.Interpreter' as a type synonym.

GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script