Better tracing for Oil

[andychu]

from #577

• Print in an unambiguous format, like CSTR QSN serialization format: parser, printer, builtin #582
• Maybe develop a better API than $PS4, analogous to Provide a prompt hook in bin/ysh #498

[andychu]

Idea: this could be replaced by "advice" around each command?

proc before-command (@argv) {
}

proc after-command (status, times Float) {   # ?
  # can record to file?
  # or maybe just pass a TSV row?
}

[andychu]

Features

• parsable so we can display an HTML version
• PID indication (SHLVL and +++ don't do this; they're confusing)
  • pipelines, subshells, processes
• Shell function stack indication

[andychu]

Fixed https://www.oilshell.org/release/0.8.7/