switch to slog.Logger for logging, for easier reuse of packages by external software

we don't want external software to include internal details like mlog.
slog.Logger is/will be the standard.

we still have mlog for its helper functions, and its handler that logs in
concise logfmt used by mox.

packages that are not meant for reuse still pass around mlog.Log for
convenience.

we use golang.org/x/exp/slog because we also support the previous Go toolchain
version. with the next Go release, we'll switch to the builtin slog.

junk/filter_test.go

@@ -32,7 +32,7 @@ func tlistdir(t *testing.T, name string) []string {
 }
 
 func TestFilter(t *testing.T) {
-	log := mlog.New("junk")
+	log := mlog.New("junk", nil)
 	params := Params{
 		Onegrams:    true,
 		Twograms:    true,