Revamp the patterns proposal. (#2338)

* Revamp the patterns proposal.

There are a large number of changes here, but structural and meaningful.

- Unify binder and matcher patterns into a single grammar.
- Always treat simple identifiers as variables in patterns.
- Change the `if (expr case pattern)` syntax.
- Change the guard syntax.
- Record patterns match only record objects.
- New patterns for relational operators, `|`, `&`, and `(...)`.
- Get rid of explicit wildcard patterns since they're redundant with
  untyped variable patterns named `_`.

* Escape `|` in Markdown table.

* Revise revise revise.

- Revamp context type schema and static type calculation.
- Remove support for late pattern variables.
- Remove support for patterns in top-level variables and fields.

* Remove type patterns from the proposal.

Author: munificent