[patterns] Remove some of the less baked parts of the proposal. (#2230)

* [patterns] Remove some of the less baked parts of the proposal.

- Get rid of declaration matcher patterns. They're syntactic sugar (on
  top of syntactic sugar) and seem to be confusing and error-prone.
  Taking them out at least for now to simplify.

- Get rid of the Destructure_n_ interface. I'd eventually like some
  protocol for user-defined positional destructuring, but it doesn't
  seem like this is it.

Removing the latter left some loose ends in record patterns around
typing and inference. I've tried to tie those up in a way that makes
sense.

* Revise.

Author: munificent

working/0546-patterns/exhaustiveness.md

@@ -367,11 +367,10 @@ into spaces:
 
 *   **Static type:** An extract space with that type and no fields.
 
-*   **Record pattern:** If it has positional fields, then an extract space
-    whose type is the corresponding `Destructure_n` type. Otherwise, an extract
-    space with type "top". Then lift the record fields to spaces on the extract
-    space. Since extract spaces only have named fields, lift positional fields
-    to implicit names like `field0`, `field1`, etc.
+*   **Record pattern:** An extract space with type "top". Then lift the record
+    fields to spaces on the extract space. Since extract spaces only have named
+    fields, lift positional fields to implicit names like `field0`, `field1`,
+    etc.
 
 *   **List pattern:** An extract space whose type is the corresponding list
     type. Element subpatterns lift to fields on the extract with names like