Updates for go1.21 rand changes

[mroth]

Work in progress

Fixes #26

[mroth]

Benchmarks using above commit, showing PickSource still quite necessary on go1.20

$ go version
go version go1.20.7 darwin/arm64
$ go test -bench=Parallel$ -cpu 8 -count 10 > go120.txt
$ benchstat -col=".name" go120.txt
goos: darwin
goarch: arm64
pkg: github.com/mroth/weightedrand/v2
            │ PickParallel  │          PickSourceParallel          │
            │    sec/op     │    sec/op     vs base                │
*/10-8        117.550n ± 1%   2.479n ±  8%  -97.89% (p=0.000 n=10)
*/100-8       125.250n ± 2%   4.143n ±  4%  -96.69% (p=0.000 n=10)
*/1000-8      131.000n ± 3%   5.942n ±  2%  -95.46% (p=0.000 n=10)
*/10000-8     154.100n ± 3%   8.178n ± 18%  -94.69% (p=0.000 n=10)
*/100000-8     180.90n ± 1%   11.18n ±  0%  -93.82% (p=0.000 n=10)
*/1000000-8    221.20n ± 1%   14.51n ±  0%  -93.44% (p=0.000 n=10)
geomean         151.1n        6.578n        -95.65%

On go1.21, the difference mostly negligible (some minor noise in the tests here due to running on laptop):

$ go1.21.0 version
go version go1.21.0 darwin/arm64
go1.21.0 test -bench=Parallel$ -cpu 8 -count 10 > go121.txt
$ benchstat -col=".name" go121.txt
goos: darwin
goarch: arm64
pkg: github.com/mroth/weightedrand/v2
            │ PickParallel │         PickSourceParallel         │
            │    sec/op    │   sec/op     vs base               │
*/10-8         2.620n ± 5%   2.449n ± 2%  -6.55% (p=0.002 n=10)
*/100-8        4.227n ± 1%   4.142n ± 1%  -2.01% (p=0.000 n=10)
*/1000-8       5.900n ± 0%   5.878n ± 0%  -0.36% (p=0.001 n=10)
*/10000-8      7.784n ± 0%   7.986n ± 0%  +2.60% (p=0.000 n=10)
*/100000-8     10.99n ± 0%   11.14n ± 0%  +1.36% (p=0.000 n=10)
*/1000000-8    14.43n ± 1%   14.52n ± 0%  +0.59% (p=0.013 n=10)
geomean        6.573n        6.522n       -0.77%

Direct comparison of go1.20 to go1.21:

$ benchstat go120.txt go121.txt
goos: darwin
goarch: arm64
pkg: github.com/mroth/weightedrand/v2
                             │   go120.txt    │              go121.txt              │
                             │     sec/op     │   sec/op     vs base                │
PickParallel/10-8              117.550n ±  1%   2.620n ± 5%  -97.77% (p=0.000 n=10)
PickParallel/100-8             125.250n ±  2%   4.227n ± 1%  -96.63% (p=0.000 n=10)
PickParallel/1000-8            131.000n ±  3%   5.900n ± 0%  -95.50% (p=0.000 n=10)
PickParallel/10000-8           154.100n ±  3%   7.784n ± 0%  -94.95% (p=0.000 n=10)
PickParallel/100000-8           180.90n ±  1%   10.99n ± 0%  -93.92% (p=0.000 n=10)
PickParallel/1000000-8          221.20n ±  1%   14.43n ± 1%  -93.48% (p=0.000 n=10)
PickSourceParallel/10-8          2.479n ±  8%   2.449n ± 2%        ~ (p=0.190 n=10)
PickSourceParallel/100-8         4.143n ±  4%   4.142n ± 1%        ~ (p=0.684 n=10)
PickSourceParallel/1000-8        5.942n ±  2%   5.878n ± 0%   -1.08% (p=0.000 n=10)
PickSourceParallel/10000-8       8.178n ± 18%   7.986n ± 0%   -2.34% (p=0.007 n=10)
PickSourceParallel/100000-8      11.18n ±  0%   11.14n ± 0%   -0.36% (p=0.005 n=10)
PickSourceParallel/1000000-8     14.51n ±  0%   14.52n ± 0%        ~ (p=0.806 n=10)
geomean                          31.52n         6.548n       -79.23%

[mroth]

A search of the dependents for this module reveal that many who use PickSource are using it incorrectly, so in order to simplify the surface area going to just go ahead and mark as deprecated as of go1.21 rather than waiting another cycle for go1.22.

There is still a potential valid use case for wanting to provide a custom randomness source, but would rather get it out of the API for now, and revisit if it can be done a more elegant way with the proposed math/rand/v2 in discussion.