This a first step in refactoring towards a v3 release.  A major semantic
version release is required in order to let us fully remove the
deprecated PickSource from our API.

Switches to using the new math/rand/v2 module.  Paving the way for the
future.

Removes PickSource method: As planned, this will remove the previously
deprecated PickSource method that uses v1 of math/rand sources. Custom
randomness source should be reintroduced in a future version using a
different methodology (e.g. setting on the Chooser instead of with each
function call).

Switches the Chooser backing from being a system int to a uint64. This
should result in defined behavior across 32 and 64 bit systems (with
the potential for some performance regressions on 32 bit systems, which
I consider an acceptable tradeoff).

Internal implementation: removes the custom searchInts binary sort that
was present for performance (ala github.com/mroth/xsort)in favor of
slices.BinarySearch which is available as of go1.21 and hits acceptable
performance as of go1.22.

goos: darwin
goarch: arm64
pkg: github.com/mroth/weightedrand/v2
                              │ v2-main.txt  │             v3-dev1.txt              │
                              │    sec/op    │    sec/op     vs base                │
NewChooser/size=1e1-8           132.7n ±  0%   132.9n ±  0%       ~ (p=0.314 n=6)
NewChooser/size=1e2-8           476.1n ±  1%   472.8n ±  0%  -0.70% (p=0.002 n=6)
NewChooser/size=1e3-8           3.406µ ±  0%   3.412µ ±  0%       ~ (p=0.379 n=6)
NewChooser/size=1e4-8           31.19µ ±  1%   31.03µ ±  0%  -0.51% (p=0.002 n=6)
NewChooser/size=1e5-8           296.6µ ±  0%   295.9µ ±  0%       ~ (p=0.394 n=6)
NewChooser/size=1e6-8           2.843m ±  1%   2.843m ±  1%       ~ (p=0.485 n=6)
NewChooser/size=1e7-8           35.83m ±  1%   35.92m ±  1%       ~ (p=0.485 n=6)
Pick/size=1e1-8                 22.49n ±  8%   20.28n ±  9%  -9.80% (p=0.015 n=6)
Pick/size=1e2-8                 35.26n ±  2%   32.82n ±  2%  -6.92% (p=0.002 n=6)
Pick/size=1e3-8                 48.41n ±  1%   45.38n ±  1%  -6.26% (p=0.002 n=6)
Pick/size=1e4-8                 63.30n ±  1%   60.23n ±  2%  -4.85% (p=0.002 n=6)
Pick/size=1e5-8                 85.92n ±  1%   82.53n ±  1%  -3.95% (p=0.002 n=6)
Pick/size=1e6-8                 111.5n ±  1%   107.3n ±  4%  -3.72% (p=0.013 n=6)
Pick/size=1e7-8                 240.7n ±  2%   233.2n ±  1%  -3.10% (p=0.002 n=6)
PickParallel/size=1e1-8         2.982n ±  6%   2.760n ±  5%  -7.43% (p=0.009 n=6)
PickParallel/size=1e2-8         4.679n ±  1%   4.360n ±  2%  -6.81% (p=0.002 n=6)
PickParallel/size=1e3-8         6.422n ±  2%   6.059n ±  1%  -5.66% (p=0.002 n=6)
PickParallel/size=1e4-8         8.463n ±  0%   8.114n ± 58%       ~ (p=0.058 n=6)
PickParallel/size=1e5-8         11.55n ±  3%   11.06n ±  0%  -4.24% (p=0.002 n=6)
PickParallel/size=1e6-8         14.98n ±  0%   14.40n ±  0%  -3.87% (p=0.002 n=6)
PickParallel/size=1e7-8         34.70n ±  0%   33.71n ±  0%  -2.82% (p=0.002 n=6)
PickSourceParallel/size=1e1-8   2.752n ± 10%
PickSourceParallel/size=1e2-8   4.369n ±  2%
PickSourceParallel/size=1e3-8   5.989n ±  1%
PickSourceParallel/size=1e4-8   7.991n ±  2%
PickSourceParallel/size=1e5-8   11.28n ±  0%
PickSourceParallel/size=1e6-8   14.59n ±  0%
PickSourceParallel/size=1e7-8   33.86n ±  0%
geomean                         120.0n         279.7n        -3.59%               ¹
¹ benchmark set differs from baseline; geomeans may not be comparable

Since we're already requiring the slices package in stdlib with this
refactor, we can utilize this newer function which should be slightly
more efficient (and has nicer ergonomics imo).

Performs roughly ~11% faster during NewChooser initialization.

goos: darwin
goarch: arm64
pkg: github.com/mroth/weightedrand/v2
                        │  v3-dev1.txt  │             v3-dev2.txt              │
                        │    sec/op     │   sec/op     vs base                 │
NewChooser/size=1e1-8     132.90n ±  0%   70.73n ± 1%  -46.78% (p=0.002 n=6)
NewChooser/size=1e2-8      472.8n ±  0%   444.1n ± 2%   -6.05% (p=0.002 n=6)
NewChooser/size=1e3-8      3.412µ ±  0%   3.333µ ± 0%   -2.30% (p=0.002 n=6)
NewChooser/size=1e4-8      31.03µ ±  0%   30.30µ ± 0%   -2.33% (p=0.002 n=6)
NewChooser/size=1e5-8      295.9µ ±  0%   291.9µ ± 1%   -1.36% (p=0.002 n=6)
NewChooser/size=1e6-8      2.843m ±  1%   2.775m ± 1%   -2.37% (p=0.002 n=6)
NewChooser/size=1e7-8      35.92m ±  1%   32.99m ± 2%   -8.16% (p=0.002 n=6)
geomean                    279.7n         36.41µ       -11.60%               ¹
¹ benchmark set differs from baseline; geomeans may not be comparable

The internal mechanics of this are a bit inelegant, since unfortunately
the global randomness source is not exported, necessitating these nil
check methods instead.

The API here needs some user feedback. I believe the majority case will
want to set this once and not on a per-call basis (cf. the deprecated
PickSource method in the previous version), but that needs be validated.