archive/zip: make benchmarks more representative

Currently zip benchmarks spend 60% in the rleBuffer code,
which is used only to test zip archive/zip itself:
    17.48s 37.02% 37.02%     18.12s 38.37%  archive/zip.(*rleBuffer).ReadAt
     9.51s 20.14% 57.16%     10.43s 22.09%  archive/zip.(*rleBuffer).Write
     9.15s 19.38% 76.54%     10.85s 22.98%  compress/flate.(*compressor).deflate

This means that benchmarks currently test performance of test helper.
Updating ReadAt/Write methods to be more performant makes benchmarks closer to real world.

name                       old time/op    new time/op    delta
CompressedZipGarbage-8       2.34ms ± 0%    2.34ms ± 1%     ~     (p=0.684 n=10+10)
Zip64Test-8                  58.1ms ± 2%    10.7ms ± 1%  -81.54%  (p=0.000 n=10+10)
Zip64TestSizes/4096-8        4.05µs ± 2%    3.65µs ± 5%   -9.96%  (p=0.000 n=9+10)
Zip64TestSizes/1048576-8      238µs ± 0%      43µs ± 0%  -82.06%  (p=0.000 n=10+10)
Zip64TestSizes/67108864-8    15.3ms ± 1%     2.6ms ± 0%  -83.12%  (p=0.000 n=10+9)

name                       old alloc/op   new alloc/op   delta
CompressedZipGarbage-8       17.9kB ±14%    16.0kB ±24%  -10.48%  (p=0.026 n=9+10)

name                       old allocs/op  new allocs/op  delta
CompressedZipGarbage-8         44.0 ± 0%      44.0 ± 0%     ~     (all equal)

Change-Id: Idfd920d0e4bed4aec2f5be84dc7e3919d9f1dd2d
Reviewed-on: https://go-review.googlesource.com/83857
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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