cmd: Add an errtrace command

[abhinav]

Adds a new errtrace binary that instruments existing Go code
with errtrace.Wrap calls.

This operates by searching for functions that return errors,
and inside those functions, wrapping all returns with errtrace.Wrap.

To allow for future usage with -toolexec or similar,
the code edits all ensure that line numbers do not change.
We may want to reconsider this behavior in the future.

This has some amount of work pending, but the PoC works.

• More testing with more corner cases
• Detect overlapping edits: this should be considered a bug
• Handle expected overlapping edits, e.g. return func() { return err }().
• Handling of statements like return foo()
  where foo() returns x, err -- this currently panics logs and skips.
  This needs helper Wrap{N} functions
  parameterized over the non-error return types,
  and that still won't handle multiple error returns
  like the rest of the program. That should be fine.
• Handling of package naming conflict:
  The code has room for using a name other than "errtrace"
  for the package, but the conflict isn't currently detected.
  This probably needs making two passes over the file
  to find a unique identifier.

[prashantv]

if the deferred function has multiple assignments to the same error value, i think this will wrap multiple times -- should we only wrap the last assignment?

don't have to address this immediately - maybe just a TODO?

[abhinav]

Don't we want all assignments to be affected, though?

defer func() {
  if cond {
    err = fmt.Errorf("cond: %w", err)
  } else {
    err = fmt.Errorf("not cond: %w", err)
  }
}

[prashantv]

Good point, better to wrap multiple times than miss one of the cases. Don't think we can do more without more advanced control flow analysis.