func MatchAny(key string, strs ...string) Exp {
	exp := make([]Exp, len(strs))
	for i, str := range strs {
		exp[i] = Match(key, str)
	}
	return Or(exp...)
}
MatchAny is an expression that evaluates to true if any of the strs are equal
to the value pointed to by key.

m := Map{"foo": "bar"}
MatchAny("foo", "bar", "baz", "cux").Eval(m) // true
MatchAny("foo", "baf", "baz", "lux").Eval(m) // false
package exp
import "strings"
type expMatch struct {
	key, str string
}
func (e expMatch) Eval(p Params) bool {
	return p.Get(e.key) == e.str
}
func (e expMatch) String() string {
	return sprintf("[%s==%s]", e.key, e.str)
}
func Match(key, str string) Exp {
	return expMatch{key, str}
}