perf: reduce unnecessary allocations in NewSHA1 and NewMD5

[anatoly-kussul]

Resubmit of accidentally closed #181 by deleting forked repo.

goos: linux
goarch: amd64
pkg: github.com/google/uuid
cpu: 12th Gen Intel(R) Core(TM) i5-12400F
                      │     old     │                 new                 │
                      │   sec/op    │   sec/op     vs base                │
MD5-12                  163.2n ± 0%   106.2n ± 0%  -34.95% (p=0.000 n=10)
SHA1-12                 176.0n ± 1%   115.8n ± 1%  -34.18% (p=0.000 n=10)

                      │     old      │                   new                   │
                      │     B/op     │    B/op     vs base                     │
MD5-12                  144.0 ± 0%       0.0 ± 0%  -100.00% (p=0.000 n=10)
SHA1-12                 168.0 ± 0%       0.0 ± 0%  -100.00% (p=0.000 n=10)

                      │     old      │                   new                   │
                      │  allocs/op   │ allocs/op   vs base                     │
MD5-12                  4.000 ± 0%     0.000 ± 0%  -100.00% (p=0.000 n=10)
SHA1-12                 4.000 ± 0%     0.000 ± 0%  -100.00% (p=0.000 n=10)