Support parallel constraint checking

This puts in place support for parallel constraint checking using
go-routines.  The implementation was surprisingly straightforward, and
offers excellent improvements.

pkg/schema/schemas.go

@@ -1,9 +1,6 @@
 package schema
 
 import (
-	"fmt"
-	"time"
-
 	tr "github.com/consensys/go-corset/pkg/trace"
 )
 
@@ -63,7 +60,6 @@ func RequiredSpillage(schema Schema) uint {
 // specifically, that means computing the actual values for any assignments.
 // Observe that assignments have to be computed in the correct order.
 func ExpandTrace(schema Schema, trace tr.Trace) error {
-	start := time.Now()
 	// Compute each assignment in turn
 	for i := schema.Assignments(); i.HasNext(); {
 		// Get ith assignment
@@ -73,7 +69,6 @@ func ExpandTrace(schema Schema, trace tr.Trace) error {
 			return err
 		}
 	}
-	fmt.Printf("Trace expansion took %.2fs\n", time.Since(start).Seconds())
 	// Done
 	return nil
 }
@@ -84,46 +79,11 @@ func ExpandTrace(schema Schema, trace tr.Trace) error {
 // which does not hold.
 //
 //nolint:revive
-func Accepts2(schema Schema, trace tr.Trace) error {
-	var err error
-	start := time.Now()
-	// Determine how many constraints
-	n := schema.Constraints().Count()
-	fmt.Printf("Beginning constraint checking (%d constraints)\n", n)
-	// Construct a channel with enough s
-	//c := make(chan error, 1)
-	// Check each constraint in turn
-	for i := schema.Constraints(); i.HasNext(); {
-		// Get ith constraint
-		ith := i.Next()
-		// Check it holds (or report an error)
-		if e := ith.Accepts(trace); e != nil {
-			err = e
-		}
-		// go func() {
-		// 	c <- ith.Accepts(trace)
-		// }()
-	}
-	// // Read responses back from each constraint.
-	// for i := uint(0); i < n; i++ {
-	// 	// Read from channel
-	// 	if e := <-c; e != nil {
-	// 		err = e
-	// 	}
-	// }
-	//
-	fmt.Printf("Constraint checking tool %.2fs\n", time.Since(start).Seconds())
-	// Success
-	return err
-}
-
 func Accepts(schema Schema, trace tr.Trace) error {
 	var err error
-	start := time.Now()
 	// Determine how many constraints
 	n := schema.Constraints().Count()
-	fmt.Printf("Beginning constraint checking (%d constraints)\n", n)
-	// Construct a channel with enough s
+	// Construct a communication channel for errors.
 	c := make(chan error, 100)
 	// Check each constraint in turn
 	for i := schema.Constraints(); i.HasNext(); {
@@ -142,8 +102,6 @@ func Accepts(schema Schema, trace tr.Trace) error {
 			err = e
 		}
 	}
-	//
-	fmt.Printf("Constraint checking tool %.2fs\n", time.Since(start).Seconds())
 	// Success
 	return err
 }