Optimize zero reads

[nirs]

Use optimized range loop[1], optimized by the compiler to single memclr
call. This dramatically speeds up zero reads.

format	compression	utilization	speedup
qcow2	-	0%	28.72
qcow2	zlib	0%	28.04
qcow2	-	50%	4.54
qcow2	zlib	50%	1.03
qcow2	-	100%	1.01
qcow2	zlib	100%	1.00

Before:

% go test -bench Read
BenchmarkRead0p/qcow2-12           14      77515735 ns/op     3462.98 MB/s      1050518 B/op        39 allocs/op
BenchmarkRead0p/qcow2_zlib-12      14      77823402 ns/op     3449.29 MB/s      1050504 B/op        39 allocs/op
BenchmarkRead50p/qcow2-12          24      48812158 ns/op     5499.36 MB/s      1181856 B/op        45 allocs/op
BenchmarkRead50p/qcow2_zlib-12      2     899659187 ns/op      298.37 MB/s    184996316 B/op     43247 allocs/op
BenchmarkRead100p/qcow2-12         61      19306020 ns/op    13904.24 MB/s      1181854 B/op        45 allocs/op
BenchmarkRead100p/qcow2_zlib-12     1    1732168542 ns/op      154.97 MB/s    368850952 B/op     86460 allocs/op

After:

% go test -bench Read
BenchmarkRead0p/qcow2-12          471       2698377 ns/op    99480.34 MB/s      1050514 B/op        39 allocs/op
BenchmarkRead0p/qcow2_zlib-12     468       2774952 ns/op    96735.15 MB/s      1050511 B/op        39 allocs/op
BenchmarkRead50p/qcow2-12         100      10735870 ns/op    25003.61 MB/s      1181854 B/op        45 allocs/op
BenchmarkRead50p/qcow2_zlib-12      2     868310583 ns/op      309.15 MB/s    185038456 B/op     43263 allocs/op
BenchmarkRead100p/qcow2-12         63      18977718 ns/op    14144.77 MB/s      1181851 B/op        45 allocs/op
BenchmarkRead100p/qcow2_zlib-12     1    1727832917 ns/op      155.36 MB/s    368886656 B/op     86471 allocs/op

Comparing with qemu-img show that we match qemu-img performance for
uncompressed version of the lima default image:

% time ./go-qcow2reader-example /tmp/test.qcow2 > /tmp/tmp.img
./go-qcow2reader-example /tmp/test.qcow2 > /tmp/tmp.img  0.06s user 0.73s system 93% cpu 0.854 total

% time qemu-img convert -O raw /tmp/test.qcow2 /tmp/tmp.img
qemu-img convert -O raw /tmp/test.qcow2 /tmp/tmp.img  0.04s user 0.70s system 98% cpu 0.756 total

[1] https://go-review.googlesource.com/c/go/+/2520

Part-of #32

Based on #35 for more correct benchmarks.

[nirs]

@AkihiroSuda current version is simpler, needs no temporary buffer, and much faster.

[AkihiroSuda]

Thanks