function printing

[porcuquine]

Functions (Funs) do not currently have a readable printed representation. That's fine, for now.

I think the default representation of a function should not include the body. This can get very long and makes looking at expressions containing functions unwieldy.

Here's what I propose.

By default:

• Functions are displayed with their argument lists and hashes (maybe a 'short hash') so it's easy to distinguish functions and gain some insight about their arity and likely identity.
• Function body is elided (...).
• The length of the closed env is displayed.

Separately, and with lower priority, we support a debug mode (ideally this can be toggled in REPL) that shows the full function body and full env. Actually, full env is likely overkill most of the time — but there is definitely a need for it sometimes. So this should perhaps be two distinct settings.