func createStandardLibrary() map[string]ast.Function {
	return map[string]ast.Function{
		"time.nano":       hilFuncTimeNano(),
		"str.truncate":    hilFuncStrTruncate(),
		"str.queryEscape": hilFuncStrQueryEscape(),
		"regexp.matches":  hilFuncRegexpMatches(),
		"counter.next":    hilFuncCounterNext(),
		"rand.base64":     hilFuncRandBase64(),
		"assert":          hilFuncAssert(),
		"json.marshal":    hilFuncJSONMarshal(),
		"map.flatten":     hilFuncMapFlatten(),
	}
}
createStandardLibrary instantiates all the functions and associates them
to their names in a lookup table for our standard library.
package interpolation
import (
	"crypto/rand"
	"encoding/base64"
	"encoding/json"
	"fmt"
	"net/url"
	"regexp"
	"sort"
	"strings"
	"sync/atomic"
	"time"
	"github.com/hashicorp/hil"
	"github.com/hashicorp/hil/ast"
	"github.com/spf13/cast"
)
var hilStandardLibrary = createStandardLibrary()