wgengine/filter/filtertype: make Match.IPProto a view

I noticed we were allocating these every time when they could just
share the same memory. Rather than document ownership, just lock it
down with a view.

I was considering doing all of the fields but decided to just do this
one first as test to see how infectious it became.  Conclusion: not
very.

Updates #cleanup (while working towards tailscale/corp#20514)

Change-Id: I8ce08519de0c9a53f20292adfbecd970fe362de0
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/deephash/deephash_test.go

@@ -27,6 +27,7 @@ import (
 	"tailscale.com/types/ipproto"
 	"tailscale.com/types/key"
 	"tailscale.com/types/ptr"
+	"tailscale.com/types/views"
 	"tailscale.com/util/deephash/testtype"
 	"tailscale.com/util/dnsname"
 	"tailscale.com/util/hashx"
@@ -353,7 +354,7 @@ func getVal() *tailscaleTypes {
 			},
 		},
 		filter.Match{
-			IPProto: []ipproto.Proto{1, 2, 3},
+			IPProto: views.SliceOf([]ipproto.Proto{1, 2, 3}),
 		},
 	}
 }