internal/deepprint: buffer writes

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The sha256 hash writer doesn't implement WriteString.
(See https://github.com/golang/go/issues/38776.)
As a consequence, we end up converting many strings to []byte.

Wrapping a bufio.Writer around the hash writer lets us
avoid these conversions by using WriteString.

Using a bufio.Writer is, perhaps surprisingly, almost as cheap as using unsafe.
The reason is that the sha256 writer does internal buffering,
but doesn't do any when handed larger writers.
Using a bufio.Writer merely shifts the data copying from one buffer
to a different one.

Using a concrete type for Print and print cuts 10% off of the execution time.

name    old time/op    new time/op    delta
Hash-8    15.3µs ± 0%    11.5µs ± 0%  -24.84%  (p=0.000 n=10+10)

name    old alloc/op   new alloc/op   delta
Hash-8    2.82kB ± 0%    1.98kB ± 0%  -29.57%  (p=0.000 n=10+10)

name    old allocs/op  new allocs/op  delta
Hash-8       140 ± 0%        82 ± 0%  -41.43%  (p=0.000 n=10+10)

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

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