Add 64-byte block loop for purego

[johejo]

It would be nice to add similar optimization for asm implementations.
For now, this PR targets only purego.

go test -bench . -count=10 -tags purego

goos: linux
goarch: amd64
pkg: github.com/cespare/xxhash/v2
cpu: AMD Ryzen 7 7735HS with Radeon Graphics
                     │ before.txt  │              after.txt              │
                     │   sec/op    │   sec/op     vs base                │
Sum64/4B-16            2.547n ± 0%   2.579n ± 1%   +1.26% (p=0.000 n=10)
Sum64/16B-16           4.509n ± 4%   4.571n ± 3%        ~ (p=0.425 n=10)
Sum64/100B-16          13.55n ± 0%   12.51n ± 1%   -7.67% (p=0.000 n=10)
Sum64/4KB-16           247.1n ± 0%   246.4n ± 0%   -0.26% (p=0.027 n=10)
Sum64/10MB-16          617.6µ ± 2%   603.7µ ± 1%   -2.26% (p=0.002 n=10)
Sum64String/4B-16      2.785n ± 0%   2.783n ± 0%        ~ (p=0.341 n=10)
Sum64String/16B-16     4.842n ± 3%   4.866n ± 2%        ~ (p=0.896 n=10)
Sum64String/100B-16    13.38n ± 1%   12.00n ± 0%  -10.32% (p=0.000 n=10)
Sum64String/4KB-16     249.7n ± 1%   246.1n ± 0%   -1.44% (p=0.000 n=10)
Sum64String/10MB-16    619.3µ ± 2%   602.7µ ± 1%   -2.68% (p=0.001 n=10)
DigestBytes/4B-16      8.023n ± 1%   7.909n ± 0%   -1.41% (p=0.000 n=10)
DigestBytes/16B-16     9.097n ± 2%   9.069n ± 2%        ~ (p=0.796 n=10)
DigestBytes/100B-16    16.27n ± 1%   16.49n ± 0%   +1.35% (p=0.000 n=10)
DigestBytes/4KB-16     303.1n ± 1%   260.8n ± 0%  -13.93% (p=0.000 n=10)
DigestBytes/10MB-16    764.3µ ± 2%   636.7µ ± 1%  -16.70% (p=0.000 n=10)
DigestString/4B-16     7.901n ± 1%   7.795n ± 1%   -1.34% (p=0.000 n=10)
DigestString/16B-16    9.064n ± 1%   8.932n ± 1%   -1.46% (p=0.002 n=10)
DigestString/100B-16   16.48n ± 1%   16.52n ± 0%        ~ (p=0.749 n=10)
DigestString/4KB-16    302.7n ± 0%   260.9n ± 0%  -13.78% (p=0.000 n=10)
DigestString/10MB-16   736.9µ ± 0%   637.8µ ± 0%  -13.45% (p=0.000 n=10)
geomean                152.8n        146.2n        -4.31%

                     │  before.txt  │              after.txt               │
                     │     B/s      │     B/s       vs base                │
Sum64/4B-16            1.463Gi ± 0%   1.445Gi ± 1%   -1.23% (p=0.000 n=10)
Sum64/16B-16           3.304Gi ± 4%   3.260Gi ± 3%        ~ (p=0.481 n=10)
Sum64/100B-16          6.868Gi ± 0%   7.442Gi ± 1%   +8.35% (p=0.000 n=10)
Sum64/4KB-16           15.08Gi ± 0%   15.12Gi ± 0%   +0.27% (p=0.019 n=10)
Sum64/10MB-16          15.08Gi ± 2%   15.43Gi ± 1%   +2.31% (p=0.002 n=10)
Sum64String/4B-16      1.338Gi ± 0%   1.339Gi ± 0%        ~ (p=0.393 n=10)
Sum64String/16B-16     3.078Gi ± 4%   3.062Gi ± 2%        ~ (p=0.912 n=10)
Sum64String/100B-16    6.963Gi ± 1%   7.765Gi ± 0%  +11.52% (p=0.000 n=10)
Sum64String/4KB-16     14.92Gi ± 1%   15.14Gi ± 0%   +1.45% (p=0.000 n=10)
Sum64String/10MB-16    15.04Gi ± 2%   15.45Gi ± 1%   +2.75% (p=0.001 n=10)
DigestBytes/4B-16      475.5Mi ± 1%   482.3Mi ± 0%   +1.43% (p=0.000 n=10)
DigestBytes/16B-16     1.638Gi ± 2%   1.643Gi ± 2%        ~ (p=0.796 n=10)
DigestBytes/100B-16    5.725Gi ± 1%   5.648Gi ± 0%   -1.34% (p=0.000 n=10)
DigestBytes/4KB-16     12.29Gi ± 1%   14.28Gi ± 0%  +16.18% (p=0.000 n=10)
DigestBytes/10MB-16    12.18Gi ± 2%   14.63Gi ± 1%  +20.04% (p=0.000 n=10)
DigestString/4B-16     482.8Mi ± 1%   489.3Mi ± 1%   +1.36% (p=0.000 n=10)
DigestString/16B-16    1.644Gi ± 1%   1.668Gi ± 1%   +1.48% (p=0.002 n=10)
DigestString/100B-16   5.651Gi ± 1%   5.639Gi ± 0%        ~ (p=0.684 n=10)
DigestString/4KB-16    12.31Gi ± 0%   14.27Gi ± 0%  +15.97% (p=0.000 n=10)
DigestString/10MB-16   12.64Gi ± 0%   14.60Gi ± 0%  +15.54% (p=0.000 n=10)
geomean                4.642Gi        4.851Gi        +4.50%

It seems to be a slight performance degradation for small inputs such as 4B.

[greatroar]

This can now become an if instead of a for. Same in writeBlocks.