env must be readable

[porcuquine]

Now that lexical environments are not alists, we need a readable representation for them. In particular, the following should not fail:

commit: 2024-01-22 7acfbb53bdadf32cd4a3397bd3e193776ffbdba1
Lurk REPL welcomes you.
user> (let ((a 1)) (current-env))
[3 iterations] => <ENV ((a . 1))>
user> <ENV ((a . 1))>
Error: Evaluation encountered an error after 1 iteration
user>