I/O environments for various utilities could be streamlined

[Richard M Kreuter]

While poking around inside TRACE to figure out https://bugs.launchpad.net/sbcl/+bug/1798991, I noticed a decent amount of repeated idioms across TRACE, the debugger, and the inspector to handle rebinding variables (mostly I/O control variables) to try to establish sensible baselines, and, at least in the debugger, a customizable interface. (src/code/debug.lisp line 789 seems to suggest that someone once considered whether io-environment binding machinery might benefit from being customizable, anyhow.)

Attached is a quick draft of an attempt to generalize and streamline the notion rebinding a bunch of variables for particular applications (and with a hook for user customization). It's written with an eye toward being something that could be worked into the bootstrap; but I haven't yet tried doing so.

If anyone has a mind to take a look and opine whether it's worth working on inclusion, let me know.