Pull request: all: add string set

Merge in DNS/adguard-home from add-strset to master

Squashed commit of the following:

commit 2500df1805dee425eafd0503983ec631de02af0b
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Apr 20 15:09:59 2021 +0300

    all: add string set

internal/aghnet/systemresolvers.go

@@ -15,9 +15,6 @@ import (
 // implementation must be safe for concurrent use.
 type HostGenFunc func() (host string)
 
-// unit is an alias for an existing map value.
-type unit = struct{}
-
 // SystemResolvers helps to work with local resolvers' addresses provided by OS.
 type SystemResolvers interface {
 	// Get returns the slice of local resolvers' addresses.