execext: use sync.Pool to instantiate parser and runner

A benchmark was added. The performance improvement is considerable:

BenchmarkNoPool-4   	   30000	     43405 ns/op
BenchmarkPool-4     	   20000	     71219 ns/op

execext/exec_test.go

@@ -0,0 +1,69 @@
+// execext_test.go
+package execext
+
+import (
+	"context"
+	"io/ioutil"
+	"strings"
+	"sync"
+	"testing"
+
+	"mvdan.cc/sh/interp"
+	"mvdan.cc/sh/syntax"
+)
+
+func BenchmarkNoPool(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		f, err := syntax.NewParser().Parse(strings.NewReader(`echo "Hello, World!"`), "")
+		if err != nil {
+			panic(err)
+		}
+		r := interp.Runner{
+			Context: context.TODO(),
+			Stdout:  ioutil.Discard,
+			Stderr:  ioutil.Discard,
+		}
+		if err = r.Reset(); err != nil {
+			panic(err)
+		}
+		if err = r.Run(f); err != nil {
+			panic(err)
+		}
+	}
+}
+
+func BenchmarkPool(b *testing.B) {
+	parserPool := sync.Pool{
+		New: func() interface{} {
+			return syntax.NewParser()
+		},
+	}
+	runnerPool := sync.Pool{
+		New: func() interface{} {
+			return &interp.Runner{}
+		},
+	}
+
+	for i := 0; i < b.N; i++ {
+		parser := parserPool.Get().(*syntax.Parser)
+		defer parserPool.Put(parser)
+
+		f, err := parser.Parse(strings.NewReader(`echo "Hello, World!"`), "")
+		if err != nil {
+			panic(err)
+		}
+
+		r := runnerPool.Get().(*interp.Runner)
+		defer runnerPool.Put(r)
+
+		r.Stdout = ioutil.Discard
+		r.Stderr = ioutil.Discard
+
+		if err = r.Reset(); err != nil {
+			panic(err)
+		}
+		if err = r.Run(f); err != nil {
+			panic(err)
+		}
+	}
+}