internal/core/adt: support unary == and != for all types

Issue #2583

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: I7e2535aa1e218aa875f44e245bddfee453c332ba
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1217011
Reviewed-by: Roger Peppe <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: mpvl

cue/parser/parser.go

@@ -1483,6 +1483,11 @@ func (p *parser) parseUnaryExpr() ast.Expr {
 	}
 
 	switch p.tok {
+	case token.EQL:
+		if !p.experiments.StructCmp {
+			break
+		}
+		fallthrough
 	case token.ADD, token.SUB, token.NOT, token.MUL,
 		token.LSS, token.LEQ, token.GEQ, token.GTR,
 		token.NEQ, token.MAT, token.NMAT: