context env stack needs rethunk

[bwo]

    pub fn env(&self) -> &Env {
        let &LoadedContext { ref ctx, ref env_stack, ..} = self;
        env_stack.last().unwrap_or(&ctx.env)
    }

does the wrong thing in combination with with_other_env: you have the right environment in self.ctx.env, but it isn't actually found. (Illustrated by ((lambda () (let (x 1) x))).)

Dunno of if the right thing to do is push the new env onto the stack and then pop it, or replace the top of the stack, or what.

[TyOverby]

I think the right thing to do is push on the stack and pop back off. Thanks for the bug report, I can't believe we didn't have a test for this!