internal/deephash: use netaddr AppendTo methods

Slightly slower, but lots less garbage.
We will recover the speed lost in a follow-up commit.

name    old time/op    new time/op    delta
Hash-8    13.5µs ± 1%    14.3µs ± 0%   +5.84%  (p=0.000 n=10+9)

name    old alloc/op   new alloc/op   delta
Hash-8    1.46kB ± 0%    0.87kB ± 0%  -40.10%  (p=0.000 n=7+10)

name    old allocs/op  new allocs/op  delta
Hash-8      43.0 ± 0%      18.0 ± 0%  -58.14%  (p=0.000 n=10+10)

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>
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