Add rand!([::AbstractRNG], ::LongSequence, [::Sampler]) methods

[jakobnissen]

I had apparently forgotten about those before.
Also refactor the old code a bit to make it easier to reason about.

[codecov]

Codecov Report

Merging #250 (fac7d40) into master (8d015fd) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #250      +/-   ##
==========================================
+ Coverage   88.75%   88.77%   +0.01%     
==========================================
  Files          30       30              
  Lines        2322     2325       +3     
==========================================
+ Hits         2061     2064       +3     
  Misses        261      261              

Flag	Coverage Δ
unittests	88.77% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
src/longsequences/randseq.jl	100.00% <100.00%> (ø)

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[jakobnissen]

This should be good to go. I'm merging as soon as tests pass - I can't imagine anyone would have anything against this change.