repl: ensure each REPL instance gets its own "context"

Before there was this weird module-scoped "context" variable which seemingly
shared the "context" of subsequent REPL instances, unless ".clear" was invoked
inside the REPL. To be proper, we need to ensure that each REPL gets its own
"context" object. I literally don't know why this "sharing" behavior was in place
before, but it was just plain wrong.