Address a bunch of records issues. (#2422)

* Address a bunch of records issues.

- Support constant records. Fix #2337.
- Support empty and one-positional-field records. Fix #2386.
- Re-add support for positional field getters Fix #2388.
- Specify the behavior of `toString()`. Fix #2389.
- Disambiguate record types in `on` clauses. Fix #2406.

* Clarify the iteration order of fields in `==`.

* Copy-edit the sections on const records and canonicalization.

There should be no meaningful changes. I just:

- Fixed some misspellings.
- Used Markdown style consistent with the rest of the doc.
- Re-worded things to, I hope, read a little more naturally.
- Removed the parenthetical on identical() in a const context because
  that felt a little too academic.

* Leave the order that positional fields are checked in == unspecified.

* Clarify that positional fields are not sugar for named fields.

Specify the evaluation order of fields.

Author: munificent