Static optimizer for constant folding (#804)

* Optimizer API with Constant Folding implementatiton
* Better logical folds and additional tests
* Add a configurable limit to constant folding

cel/BUILD.bazel

@@ -10,9 +10,11 @@ go_library(
         "cel.go",
         "decls.go",
         "env.go",
+        "folding.go",
         "io.go",
         "library.go",
         "macro.go",
+        "optimizer.go",
         "options.go",
         "program.go",
         "validator.go",
@@ -56,6 +58,7 @@ go_test(
         "cel_test.go",
         "decls_test.go",
         "env_test.go",
+        "folding_test.go",
         "io_test.go",
         "validator_test.go",
     ],

cel/env.go

@@ -43,6 +43,9 @@ type Ast struct {
 }
 
 // Expr returns the proto serializable instance of the parsed/checked expression.
+//
+// Deprecated: prefer cel.AstToCheckedExpr() or cel.AstToParsedExpr() and call GetExpr()
+// the result instead.
 func (ast *Ast) Expr() *exprpb.Expr {
 	if ast == nil {
 		return nil
@@ -221,6 +224,11 @@ func (e *Env) Check(ast *Ast) (*Ast, *Issues) {
 		source: ast.Source(),
 		impl:   checked}
 
+	// Avoid creating a validator config if it's not needed.
+	if len(e.validators) == 0 {
+		return ast, nil
+	}
+
 	// Generate a validator configuration from the set of configured validators.
 	vConfig := newValidatorConfig()
 	for _, v := range e.validators {