Revive esdebug

[jpco]

The esdebug script is a cool and useful bit of es, but it has been broken for a while.

This PR makes changes imposed by language drift over time so that esdebug can again be run like before.

• Replace #! /bin/es with #!/usr/local/bin/es, reflecting the standard install location today
• Replace line call with %read
• Replace return with result where the exception behavior isn't necessary
• Replace $&while with a lexically-captured while
• Replace dispatch param to %interactive-loop with dynamically-defined fn-%dispatch
• Handle exit's change to an exception

The script is still pretty old and crufty, though, and there's quite a bit more that could be done as a follow-up to clean this up and make it more useful if we wanted to actually, say, distribute it with the shell.

• Use lexically-bound %seq and if to get rid of the primitive madness
• Lean on lexical binding harder in order to better isolate debugger state
• Improve behavior around settor functions and trap/trace/watch
• Test it and fix the bugs that are still too easy to run into
• Document it, since debuggers tend to be complicated to use

Because I can't leave well enough alone, here are also some fancier things that could be done if runtime support were added:

• A $&readline primitive (see Up for discussion: User-provided commands for reading shell input #178) could make it nicer to use the debugger's prompt
• A general %set hook (see Another bad idea: $&whatis, settor variables, and a new primitive #92) would make more robust tracing possible, especially with variables that already have settors
• More customizable argument parsing related behavior (see Up for discussion: Move the "main runtime" of es out of C and into es script #79) might make it possible to go into "debugger mode" with . -D script.es, or to even just drop into the debugger in the middle of a running shell session with something like runflags = $runflags debug