internal/core/adt: use per-field comprehensions

This introduces the new comprehension algorithm. It is
described in internal/core/adt/comprehension.go.

Test file changes:
- comp*.txtar: note that the permanent errors are now
incomplete errors (cycle errors, to be precise), as these
are recoverable by making a configuration more specific.
- Some orderings have changed as a result of the algorithmic
changes. Note that there are currently no ordering
guarantees.
- Some cycle errors disappeared. This is okay as long  as
at least one of the values in the cycle has the error (see the
021_delayed_constraints_failure test, for instance).
- Some error paths now reflect the field to which the
comprehension was "pushed", rather than the original.
- This nothwithstanding, the comprehension error should
also be recorded at the original location.

Fixes #1130
Fixes #1083
Fixes #1732

Change-Id: I5baeef5128ab9e7e28055bf084a43618cfc73bb8
Signed-off-by: Marcel van Lohuizen <[email protected]>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/529524
Reviewed-by: Marcel van Lohuizen <[email protected]>

Author: mpvl

cue/syntax_test.go

@@ -84,7 +84,7 @@ func TestSyntax(t *testing.T) {
 		options: o(cue.ResolveReferences(true)),
 		out: `
 {
-	b: _|_ // #List.next: structural cycle
+	b: _|_ // #List.next: structural cycle (and 1 more errors)
 }`,
 	}, {
 		name: "resolveReferences",

cue/testdata/comprehensions/closed.txtar

@@ -36,6 +36,9 @@ disallowed: {
 Errors:
 disallowed.vErr.d: field not allowed: d:
     ./in.cue:26:6
+    ./in.cue:27:3
+    ./in.cue:27:12
+    ./in.cue:28:4
     ./in.cue:32:8
     ./in.cue:32:14
 
@@ -74,6 +77,9 @@ Result:
       d: (_|_){
         // [eval] disallowed.vErr.d: field not allowed: d:
         //     ./in.cue:26:6
+        //     ./in.cue:27:3
+        //     ./in.cue:27:12
+        //     ./in.cue:28:4
         //     ./in.cue:32:8
         //     ./in.cue:32:14
       }

cue/testdata/comprehensions/iferror.txtar

@@ -148,8 +148,8 @@ Result:
   }
   useDefault: (struct){
     a: (struct){
-      x: (string){ |(*(string){ "foo" }, (string){ string }) }
       y: (string){ |(*(string){ "foo" }, (string){ string }) }
+      x: (string){ |(*(string){ "foo" }, (string){ string }) }
     }
     issue809: (struct){
       #A: (#struct){