[bugfix] Sequentially issue read requests in ReadAt the same as WriteTo

[puellanivis]

Copy in the sequential read functionality for WriteTo into ReadAt.

name                    old time/op    new time/op    delta
Read1k-8                  47.4ms ± 1%    47.4ms ± 0%    ~     (p=1.000 n=10+10)
Read16k-8                 9.94ms ± 3%    9.86ms ± 3%    ~     (p=0.393 n=10+10)
Read32k-8                 7.51ms ± 1%    7.46ms ± 2%    ~     (p=0.280 n=10+10)
Read128k-8                4.87ms ± 1%    4.78ms ± 2%  -1.76%  (p=0.000 n=9+9)
Read512k-8                3.95ms ± 2%    3.86ms ± 2%  -2.44%  (p=0.000 n=9+10)
Read1MiB-8                4.01ms ± 1%    3.92ms ± 1%  -2.28%  (p=0.000 n=9+9)
Read4MiB-8                4.43ms ± 1%    4.36ms ± 0%  -1.74%  (p=0.000 n=10+7)
Read4MiBDelay10Msec-8     69.6ms ± 1%    69.5ms ± 0%    ~     (p=0.739 n=10+10)
Read4MiBDelay50Msec-8      311ms ± 0%     311ms ± 0%    ~     (p=0.739 n=10+10)
Read4MiBDelay150Msec-8     912ms ± 0%     912ms ± 0%    ~     (p=0.796 n=10+10)

name                    old speed      new speed      delta
Read1k-8                 221MB/s ± 1%   221MB/s ± 0%    ~     (p=1.000 n=10+10)
Read16k-8               1.06GB/s ± 3%  1.06GB/s ± 3%    ~     (p=0.393 n=10+10)
Read32k-8               1.40GB/s ± 1%  1.41GB/s ± 2%    ~     (p=0.280 n=10+10)
Read128k-8              2.16GB/s ± 1%  2.19GB/s ± 2%  +1.80%  (p=0.000 n=9+9)
Read512k-8              2.65GB/s ± 2%  2.72GB/s ± 2%  +2.50%  (p=0.000 n=9+10)
Read1MiB-8              2.62GB/s ± 1%  2.68GB/s ± 1%  +2.34%  (p=0.000 n=9+9)
Read4MiB-8              2.36GB/s ± 1%  2.41GB/s ± 0%  +1.77%  (p=0.000 n=10+7)
Read4MiBDelay10Msec-8    151MB/s ± 1%   151MB/s ± 0%    ~     (p=0.671 n=10+10)
Read4MiBDelay50Msec-8   33.7MB/s ± 0%  33.7MB/s ± 0%    ~     (p=0.842 n=10+10)
Read4MiBDelay150Msec-8  11.5MB/s ± 0%  11.5MB/s ± 0%    ~     (p=1.000 n=10+10)

name                    old alloc/op   new alloc/op   delta
Read1k-8                  6.99MB ± 0%    6.99MB ± 0%    ~     (p=0.328 n=9+9)
Read16k-8                 5.99MB ± 0%    5.99MB ± 0%    ~     (p=0.929 n=10+10)
Read32k-8                 6.63MB ± 0%    6.63MB ± 0%    ~     (p=0.439 n=10+10)
Read128k-8                6.77MB ± 0%    6.77MB ± 0%  +0.00%  (p=0.022 n=10+10)
Read512k-8                7.25MB ± 0%    7.25MB ± 0%  +0.02%  (p=0.000 n=10+10)
Read1MiB-8                7.91MB ± 0%    7.91MB ± 0%  +0.01%  (p=0.000 n=10+10)
Read4MiB-8                10.5MB ± 0%    10.5MB ± 0%  +0.01%  (p=0.000 n=9+10)
Read4MiBDelay10Msec-8     10.5MB ± 0%    10.5MB ± 0%  +0.02%  (p=0.000 n=10+10)
Read4MiBDelay50Msec-8     10.5MB ± 0%    10.5MB ± 0%  +0.01%  (p=0.005 n=10+10)
Read4MiBDelay150Msec-8    10.5MB ± 0%    10.5MB ± 0%  +0.02%  (p=0.000 n=10+9)

name                    old allocs/op  new allocs/op  delta
Read1k-8                   30.7k ± 0%     30.7k ± 0%    ~     (all equal)
Read16k-8                  1.94k ± 0%     1.94k ± 0%    ~     (all equal)
Read32k-8                    982 ± 0%       982 ± 0%    ~     (all equal)
Read128k-8                 1.25k ± 0%     1.25k ± 0%  +0.04%  (p=0.015 n=9+10)
Read512k-8                 1.14k ± 0%     1.14k ± 0%  +0.06%  (p=0.019 n=10+10)
Read1MiB-8                 1.22k ± 0%     1.21k ± 0%  -0.53%  (p=0.000 n=10+10)
Read4MiB-8                 1.32k ± 0%     1.32k ± 0%  +0.55%  (p=0.000 n=9+9)
Read4MiBDelay10Msec-8      1.57k ± 1%     1.58k ± 0%  +0.66%  (p=0.000 n=10+10)
Read4MiBDelay50Msec-8      1.58k ± 1%     1.58k ± 1%    ~     (p=0.066 n=10+10)
Read4MiBDelay150Msec-8     1.58k ± 1%     1.59k ± 1%  +1.15%  (p=0.002 n=10+9)